Django Model field reference

=====================
Model field reference
=====================

.. module:: django.db.models.fields
:synopsis: Built-in field types.

.. currentmodule:: django.db.models

This document contains all the API references of :class:Field including the
field options_ and field types_ Django offers.

.. seealso::

If the built-in fields don't do the trick, you can try `django-localflavor
`_ (`documentation
`_), which contains assorted
pieces of code that are useful for particular countries and cultures.

Also, you can easily :doc:`write your own custom model fields
`.

.. note::

Technically, these models are defined in :mod:`django.db.models.fields`, but
for convenience they're imported into :mod:`django.db.models`; the standard
convention is to use ``from django.db import models`` and refer to fields as
``models.Field``.

.. _common-model-field-options:

Field options

The following arguments are available to all field types. All are optional.

null

.. attribute:: Field.null

If True, Django will store empty values as NULL in the database. Default
is False.

Avoid using :attr:~Field.null on string-based fields such as
:class:CharField and :class:TextField. If a string-based field has
null=True, that means it has two possible values for "no data": NULL,
and the empty string. In most cases, it's redundant to have two possible values
for "no data;" the Django convention is to use the empty string, not
NULL. One exception is when a :class:CharField has both unique=True
and blank=True set. In this situation, null=True is required to avoid
unique constraint violations when saving multiple objects with blank values.

For both string-based and non-string-based fields, you will also need to
set blank=True if you wish to permit empty values in forms, as the
:attr:~Field.null parameter only affects database storage
(see :attr:~Field.blank).

.. note::

When using the Oracle database backend, the value ``NULL`` will be stored to
denote the empty string regardless of this attribute.

If you want to accept :attr:~Field.null values with :class:BooleanField,
use :class:NullBooleanField instead.

blank

.. attribute:: Field.blank

If True, the field is allowed to be blank. Default is False.

Note that this is different than :attr:~Field.null. :attr:~Field.null is
purely database-related, whereas :attr:~Field.blank is validation-related. If
a field has blank=True, form validation will allow entry of an empty value.
If a field has blank=False, the field will be required.

.. _field-choices:

choices

.. attribute:: Field.choices

An iterable (e.g., a list or tuple) consisting itself of iterables of exactly
two items (e.g. [(A, B), (A, B) ...]) to use as choices for this field. If
this is given, the default form widget will be a select box with these choices
instead of the standard text field.

The first element in each tuple is the actual value to be set on the model,
and the second element is the human-readable name. For example::

YEAR_IN_SCHOOL_CHOICES = (
    ('FR', 'Freshman'),
    ('SO', 'Sophomore'),
    ('JR', 'Junior'),
    ('SR', 'Senior'),
)

Generally, it's best to define choices inside a model class, and to
define a suitably-named constant for each value::

from django.db import models

class Student(models.Model):
    FRESHMAN = 'FR'
    SOPHOMORE = 'SO'
    JUNIOR = 'JR'
    SENIOR = 'SR'
    YEAR_IN_SCHOOL_CHOICES = (
        (FRESHMAN, 'Freshman'),
        (SOPHOMORE, 'Sophomore'),
        (JUNIOR, 'Junior'),
        (SENIOR, 'Senior'),
    )
    year_in_school = models.CharField(
        max_length=2,
        choices=YEAR_IN_SCHOOL_CHOICES,
        default=FRESHMAN,
    )

    def is_upperclass(self):
        return self.year_in_school in (self.JUNIOR, self.SENIOR)

Though you can define a choices list outside of a model class and then
refer to it, defining the choices and names for each choice inside the
model class keeps all of that information with the class that uses it,
and makes the choices easy to reference (e.g, Student.SOPHOMORE
will work anywhere that the Student model has been imported).

You can also collect your available choices into named groups that can
be used for organizational purposes::

MEDIA_CHOICES = (
    ('Audio', (
            ('vinyl', 'Vinyl'),
            ('cd', 'CD'),
        )
    ),
    ('Video', (
            ('vhs', 'VHS Tape'),
            ('dvd', 'DVD'),
        )
    ),
    ('unknown', 'Unknown'),
)

The first element in each tuple is the name to apply to the group. The
second element is an iterable of 2-tuples, with each 2-tuple containing
a value and a human-readable name for an option. Grouped options may be
combined with ungrouped options within a single list (such as the
unknown option in this example).

For each model field that has :attr:~Field.choices set, Django will add a
method to retrieve the human-readable name for the field's current value. See
:meth:~django.db.models.Model.get_FOO_display in the database API
documentation.

Note that choices can be any iterable object -- not necessarily a list or tuple.
This lets you construct choices dynamically. But if you find yourself hacking
:attr:~Field.choices to be dynamic, you're probably better off using a proper
database table with a :class:ForeignKey. :attr:~Field.choices is meant for
static data that doesn't change much, if ever.

Unless :attr:blank=False is set on the field along with a
:attr:~Field.default then a label containing "---------" will be rendered
with the select box. To override this behavior, add a tuple to choices
containing None; e.g. (None, 'Your String For Display').
Alternatively, you can use an empty string instead of None where this makes
sense - such as on a :class:~django.db.models.CharField.

db_column

.. attribute:: Field.db_column

The name of the database column to use for this field. If this isn't given,
Django will use the field's name.

If your database column name is an SQL reserved word, or contains
characters that aren't allowed in Python variable names -- notably, the
hyphen -- that's OK. Django quotes column and table names behind the
scenes.

db_index

.. attribute:: Field.db_index

If True, a database index will be created for this field.

db_tablespace

.. attribute:: Field.db_tablespace

The name of the :doc:database tablespace to use for
this field's index, if this field is indexed. The default is the project's
:setting:DEFAULT_INDEX_TABLESPACE setting, if set, or the
:attr:~Options.db_tablespace of the model, if any. If the backend doesn't
support tablespaces for indexes, this option is ignored.

default

.. attribute:: Field.default

The default value for the field. This can be a value or a callable object. If
callable it will be called every time a new object is created.

The default can't be a mutable object (model instance, list, set, etc.),
as a reference to the same instance of that object would be used as the default
value in all new model instances. Instead, wrap the desired default in a
callable. For example, if you want to specify a default dict for
:class:~django.contrib.postgres.fields.JSONField, use a function::

def contact_default():
    return {"email": "[email protected]"}

contact_info = JSONField("ContactInfo", default=contact_default)

lambda\s can't be used for field options like default because they
can't be :ref:serialized by migrations . See that
documentation for other caveats.

For fields like :class:ForeignKey that map to model instances, defaults
should be the value of the field they reference (pk unless
:attr:~ForeignKey.to_field is set) instead of model instances.

The default value is used when new model instances are created and a value
isn't provided for the field. When the field is a primary key, the default is
also used when the field is set to None.

editable

.. attribute:: Field.editable

If False, the field will not be displayed in the admin or any other
:class:~django.forms.ModelForm. They are also skipped during :ref:model validation . Default is True.

error_messages

.. attribute:: Field.error_messages

The error_messages argument lets you override the default messages that the
field will raise. Pass in a dictionary with keys matching the error messages you
want to override.

Error message keys include null, blank, invalid, invalid_choice,
unique, and unique_for_date. Additional error message keys are
specified for each field in the Field types_ section below.

help_text

.. attribute:: Field.help_text

Extra "help" text to be displayed with the form widget. It's useful for
documentation even if your field isn't used on a form.

Note that this value is not HTML-escaped in automatically-generated
forms. This lets you include HTML in :attr:~Field.help_text if you so
desire. For example::

help_text="Please use the following format: YYYY-MM-DD."

Alternatively you can use plain text and
django.utils.html.escape() to escape any HTML special characters. Ensure
that you escape any help text that may come from untrusted users to avoid a
cross-site scripting attack.

primary_key

.. attribute:: Field.primary_key

If True, this field is the primary key for the model.

If you don't specify primary_key=True for any field in your model, Django
will automatically add an :class:AutoField to hold the primary key, so you
don't need to set primary_key=True on any of your fields unless you want to
override the default primary-key behavior. For more, see
:ref:automatic-primary-key-fields.

primary_key=True implies :attr:null=False and
:attr:unique=True . Only one primary key is allowed on an
object.

The primary key field is read-only. If you change the value of the primary
key on an existing object and then save it, a new object will be created
alongside the old one.

unique

.. attribute:: Field.unique

If True, this field must be unique throughout the table.

This is enforced at the database level and by model validation. If
you try to save a model with a duplicate value in a :attr:~Field.unique
field, a :exc:django.db.IntegrityError will be raised by the model's
:meth:~django.db.models.Model.save method.

This option is valid on all field types except :class:ManyToManyField,
:class:OneToOneField, and :class:FileField.

Note that when unique is True, you don't need to specify
:attr:~Field.db_index, because unique implies the creation of an index.

unique_for_date

.. attribute:: Field.unique_for_date

Set this to the name of a :class:DateField or :class:DateTimeField to
require that this field be unique for the value of the date field.

For example, if you have a field title that has
unique_for_date="pub_date", then Django wouldn't allow the entry of two
records with the same title and pub_date.

Note that if you set this to point to a :class:DateTimeField, only the date
portion of the field will be considered. Besides, when :setting:USE_TZ is
True, the check will be performed in the :ref:current time zone at the time the object gets saved.

This is enforced by :meth:Model.validate_unique() during model validation
but not at the database level. If any :attr:~Field.unique_for_date constraint
involves fields that are not part of a :class:~django.forms.ModelForm (for
example, if one of the fields is listed in exclude or has
:attr:editable=False), :meth:Model.validate_unique() will
skip validation for that particular constraint.

unique_for_month

.. attribute:: Field.unique_for_month

Like :attr:~Field.unique_for_date, but requires the field to be unique with
respect to the month.

unique_for_year

.. attribute:: Field.unique_for_year

Like :attr:~Field.unique_for_date and :attr:~Field.unique_for_month.

verbose_name

.. attribute:: Field.verbose_name

A human-readable name for the field. If the verbose name isn't given, Django
will automatically create it using the field's attribute name, converting
underscores to spaces. See :ref:Verbose field names .

validators

.. attribute:: Field.validators

A list of validators to run for this field. See the :doc:validators documentation for more information.

Registering and fetching lookups
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Field implements the :ref:lookup registration API .
The API can be used to customize which lookups are available for a field class, and
how lookups are fetched from a field.

.. _model-field-types:

Field types

.. currentmodule:: django.db.models

AutoField

.. class:: AutoField(**options)

An :class:IntegerField that automatically increments
according to available IDs. You usually won't need to use this directly; a
primary key field will automatically be added to your model if you don't specify
otherwise. See :ref:automatic-primary-key-fields.

BigAutoField

.. class:: BigAutoField(**options)

.. versionadded:: 1.10

A 64-bit integer, much like an :class:AutoField except that it is
guaranteed to fit numbers from 1 to 9223372036854775807.

BigIntegerField

.. class:: BigIntegerField(**options)

A 64-bit integer, much like an :class:IntegerField except that it is
guaranteed to fit numbers from -9223372036854775808 to
9223372036854775807. The default form widget for this field is a
:class:~django.forms.TextInput.

BinaryField

.. class:: BinaryField(**options)

A field to store raw binary data. It only supports bytes assignment. Be
aware that this field has limited functionality. For example, it is not possible
to filter a queryset on a BinaryField value. It is also not possible to
include a BinaryField in a :class:~django.forms.ModelForm.

.. admonition:: Abusing BinaryField

Although you might think about storing files in the database, consider that
it is bad design in 99% of the cases. This field is *not* a replacement for
proper :doc:`static files ` handling.

BooleanField

.. class:: BooleanField(**options)

A true/false field.

The default form widget for this field is a
:class:~django.forms.CheckboxInput.

If you need to accept :attr:~Field.null values then use
:class:NullBooleanField instead.

The default value of BooleanField is None when :attr:Field.default
isn't defined.

CharField

.. class:: CharField(max_length=None, **options)

A string field, for small- to large-sized strings.

For large amounts of text, use :class:~django.db.models.TextField.

The default form widget for this field is a :class:~django.forms.TextInput.

:class:CharField has one extra required argument:

.. attribute:: CharField.max_length

The maximum length (in characters) of the field. The max_length is enforced
at the database level and in Django's validation.

.. note::

If you are writing an application that must be portable to multiple
database backends, you should be aware that there are restrictions on
``max_length`` for some backends. Refer to the :doc:`database backend
notes ` for details.

.. admonition:: MySQL users

If you are using this field with MySQLdb 1.2.2 and the ``utf8_bin``
collation (which is *not* the default), there are some issues to be aware
of. Refer to the :ref:`MySQL database notes ` for
details.

CommaSeparatedIntegerField

.. class:: CommaSeparatedIntegerField(max_length=None, **options)

.. deprecated:: 1.9

This field is deprecated in favor of :class:`~django.db.models.CharField`
with ``validators=[``\ :func:`validate_comma_separated_integer_list
`\ ``]``.

A field of integers separated by commas. As in :class:CharField, the
:attr:~CharField.max_length argument is required and the note about database
portability mentioned there should be heeded.

DateField

.. class:: DateField(auto_now=False, auto_now_add=False, **options)

A date, represented in Python by a datetime.date instance. Has a few extra,
optional arguments:

.. attribute:: DateField.auto_now

Automatically set the field to now every time the object is saved. Useful
for "last-modified" timestamps. Note that the current date is *always*
used; it's not just a default value that you can override.

The field is only automatically updated when calling :meth:`Model.save()
`. The field isn't updated when making updates
to other fields in other ways such as :meth:`QuerySet.update()
`, though you can specify a custom
value for the field in an update like that.

.. attribute:: DateField.auto_now_add

Automatically set the field to now when the object is first created. Useful
for creation of timestamps. Note that the current date is *always* used;
it's not just a default value that you can override. So even if you
set a value for this field when creating the object, it will be ignored.
If you want to be able to modify this field, set the following instead of
``auto_now_add=True``:

* For :class:`DateField`: ``default=date.today`` - from
  :meth:`datetime.date.today`
* For :class:`DateTimeField`: ``default=timezone.now`` - from
  :func:`django.utils.timezone.now`

The default form widget for this field is a
:class:~django.forms.TextInput. The admin adds a JavaScript calendar,
and a shortcut for "Today". Includes an additional invalid_date error
message key.

The options auto_now_add, auto_now, and default are mutually exclusive.
Any combination of these options will result in an error.

.. note::
As currently implemented, setting auto_now or auto_now_add to
True will cause the field to have editable=False and blank=True
set.

.. note::
The auto_now and auto_now_add options will always use the date in
the :ref:default timezone at the moment of
creation or update. If you need something different, you may want to
consider simply using your own callable default or overriding save()
instead of using auto_now or auto_now_add; or using a
DateTimeField instead of a DateField and deciding how to handle the
conversion from datetime to date at display time.

DateTimeField

.. class:: DateTimeField(auto_now=False, auto_now_add=False, **options)

A date and time, represented in Python by a datetime.datetime instance.
Takes the same extra arguments as :class:DateField.

The default form widget for this field is a single
:class:~django.forms.TextInput. The admin uses two separate
:class:~django.forms.TextInput widgets with JavaScript shortcuts.

DecimalField

.. class:: DecimalField(max_digits=None, decimal_places=None, **options)

A fixed-precision decimal number, represented in Python by a
:class:~decimal.Decimal instance. Has two required arguments:

.. attribute:: DecimalField.max_digits

The maximum number of digits allowed in the number. Note that this number
must be greater than or equal to ``decimal_places``.

.. attribute:: DecimalField.decimal_places

The number of decimal places to store with the number.

For example, to store numbers up to 999 with a resolution of 2 decimal
places, you'd use::

models.DecimalField(..., max_digits=5, decimal_places=2)

And to store numbers up to approximately one billion with a resolution of 10
decimal places::

models.DecimalField(..., max_digits=19, decimal_places=10)

The default form widget for this field is a :class:~django.forms.NumberInput
when :attr:~django.forms.Field.localize is False or
:class:~django.forms.TextInput otherwise.

.. note::

For more information about the differences between the
:class:`FloatField` and :class:`DecimalField` classes, please
see :ref:`FloatField vs. DecimalField `.

DurationField

.. class:: DurationField(**options)

A field for storing periods of time - modeled in Python by
:class:~python:datetime.timedelta. When used on PostgreSQL, the data type
used is an interval and on Oracle the data type is INTERVAL DAY(9) TO SECOND(6). Otherwise a bigint of microseconds is used.

.. note::

Arithmetic with ``DurationField`` works in most cases. However on all
databases other than PostgreSQL, comparing the value of a ``DurationField``
to arithmetic on ``DateTimeField`` instances will not work as expected.

EmailField

.. class:: EmailField(max_length=254, **options)

A :class:CharField that checks that the value is a valid email address. It
uses :class:~django.core.validators.EmailValidator to validate the input.

FileField

.. class:: FileField(upload_to=None, max_length=100, **options)

A file-upload field.

.. note::
The primary_key and unique arguments are not supported, and will
raise a TypeError if used.

Has two optional arguments:

.. attribute:: FileField.upload_to

This attribute provides a way of setting the upload directory and file name,
and can be set in two ways. In both cases, the value is passed to the
:meth:`Storage.save() ` method.

If you specify a string value, it may contain :func:`~time.strftime`
formatting, which will be replaced by the date/time of the file upload (so
that uploaded files don't fill up the given directory). For example::

    class MyModel(models.Model):
        # file will be uploaded to MEDIA_ROOT/uploads
        upload = models.FileField(upload_to='uploads/')
        # or...
        # file will be saved to MEDIA_ROOT/uploads/2015/01/30
        upload = models.FileField(upload_to='uploads/%Y/%m/%d/')

If you are using the default
:class:`~django.core.files.storage.FileSystemStorage`, the string value
will be appended to your :setting:`MEDIA_ROOT` path to form the location on
the local filesystem where uploaded files will be stored. If you are using
a different storage, check that storage's documentation to see how it
handles ``upload_to``.

``upload_to`` may also be a callable, such as a function. This will be
called to obtain the upload path, including the filename. This callable must
accept two arguments and return a Unix-style path (with forward slashes)
to be passed along to the storage system. The two arguments are:

======================  ===============================================
Argument                Description
======================  ===============================================
``instance``            An instance of the model where the
                        ``FileField`` is defined. More specifically,
                        this is the particular instance where the
                        current file is being attached.

                        In most cases, this object will not have been
                        saved to the database yet, so if it uses the
                        default ``AutoField``, *it might not yet have a
                        value for its primary key field*.

``filename``            The filename that was originally given to the
                        file. This may or may not be taken into account
                        when determining the final destination path.
======================  ===============================================

For example::

    def user_directory_path(instance, filename):
        # file will be uploaded to MEDIA_ROOT/user_/
        return 'user_{0}/{1}'.format(instance.user.id, filename)

    class MyModel(models.Model):
        upload = models.FileField(upload_to=user_directory_path)

.. attribute:: FileField.storage

A storage object, which handles the storage and retrieval of your
files. See :doc:`/topics/files` for details on how to provide this object.

The default form widget for this field is a
:class:~django.forms.ClearableFileInput.

Using a :class:FileField or an :class:ImageField (see below) in a model
takes a few steps:

1. In your settings file, you'll need to define :setting:MEDIA_ROOT as the
   full path to a directory where you'd like Django to store uploaded files.
   (For performance, these files are not stored in the database.) Define
   :setting:MEDIA_URL as the base public URL of that directory. Make sure
   that this directory is writable by the Web server's user account.

2. Add the :class:FileField or :class:ImageField to your model, defining
   the :attr:~FileField.upload_to option to specify a subdirectory of
   :setting:MEDIA_ROOT to use for uploaded files.

3. All that will be stored in your database is a path to the file
   (relative to :setting:MEDIA_ROOT). You'll most likely want to use the
   convenience :attr:~django.db.models.fields.files.FieldFile.url attribute
   provided by Django. For example, if your :class:ImageField is called
   mug_shot, you can get the absolute path to your image in a template with
   {{ object.mug_shot.url }}.

For example, say your :setting:MEDIA_ROOT is set to '/home/media', and
:attr:~FileField.upload_to is set to 'photos/%Y/%m/%d'. The '%Y/%m/%d'
part of :attr:~FileField.upload_to is :func:~time.strftime formatting;
'%Y' is the four-digit year, '%m' is the two-digit month and '%d' is
the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in
the directory /home/media/photos/2007/01/15.

If you wanted to retrieve the uploaded file's on-disk filename, or the file's
size, you could use the :attr:~django.core.files.File.name and
:attr:~django.core.files.File.size attributes respectively; for more
information on the available attributes and methods, see the
:class:~django.core.files.File class reference and the :doc:/topics/files
topic guide.

.. note::
The file is saved as part of saving the model in the database, so the actual
file name used on disk cannot be relied on until after the model has been
saved.

The uploaded file's relative URL can be obtained using the
:attr:~django.db.models.fields.files.FieldFile.url attribute. Internally,
this calls the :meth:~django.core.files.storage.Storage.url method of the
underlying :class:~django.core.files.storage.Storage class.

.. _file-upload-security:

Note that whenever you deal with uploaded files, you should pay close attention
to where you're uploading them and what type of files they are, to avoid
security holes. Validate all uploaded files so that you're sure the files are
what you think they are. For example, if you blindly let somebody upload files,
without validation, to a directory that's within your Web server's document
root, then somebody could upload a CGI or PHP script and execute that script by
visiting its URL on your site. Don't allow that.

Also note that even an uploaded HTML file, since it can be executed by the
browser (though not by the server), can pose security threats that are
equivalent to XSS or CSRF attacks.

:class:FileField instances are created in your database as varchar
columns with a default max length of 100 characters. As with other fields, you
can change the maximum length using the :attr:~CharField.max_length argument.

FileField and FieldFile
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. currentmodule:: django.db.models.fields.files

.. class:: FieldFile

When you access a :class:~django.db.models.FileField on a model, you are
given an instance of :class:FieldFile as a proxy for accessing the underlying
file.

The API of :class:FieldFile mirrors that of :class:~django.core.files.File,
with one key difference: The object wrapped by the class is not necessarily a
wrapper around Python's built-in file object. Instead, it is a wrapper around
the result of the :attr:Storage.open()
method, which may be a :class:~django.core.files.File object, or it may be a
custom storage's implementation of the :class:~django.core.files.File API.

In addition to the API inherited from
:class:~django.core.files.File such as :meth:~django.core.files.File.read
and :meth:~django.core.files.File.write, :class:FieldFile includes several
methods that can be used to interact with the underlying file:

.. warning::

Two methods of this class, :meth:`~FieldFile.save` and
:meth:`~FieldFile.delete`, default to saving the model object of the
associated ``FieldFile`` in the database.

.. attribute:: FieldFile.name

The name of the file including the relative path from the root of the
:class:~django.core.files.storage.Storage of the associated
:class:~django.db.models.FileField.

.. attribute:: FieldFile.size

The result of the underlying :attr:Storage.size() method.

.. attribute:: FieldFile.url

A read-only property to access the file's relative URL by calling the
:meth:~django.core.files.storage.Storage.url method of the underlying
:class:~django.core.files.storage.Storage class.

.. method:: FieldFile.open(mode='rb')

Opens or reopens the file associated with this instance in the specified
mode. Unlike the standard Python open() method, it doesn't return a
file descriptor.

Since the underlying file is opened implicitly when accessing it, it may be
unnecessary to call this method except to reset the pointer to the underlying
file or to change the mode.

.. method:: FieldFile.close()

Behaves like the standard Python file.close() method and closes the file
associated with this instance.

.. method:: FieldFile.save(name, content, save=True)

This method takes a filename and file contents and passes them to the storage
class for the field, then associates the stored file with the model field.
If you want to manually associate file data with
:class:~django.db.models.FileField instances on your model, the save()
method is used to persist that file data.

Takes two required arguments: name which is the name of the file, and
content which is an object containing the file's contents. The
optional save argument controls whether or not the model instance is
saved after the file associated with this field has been altered. Defaults to
True.

Note that the content argument should be an instance of
:class:django.core.files.File, not Python's built-in file object.
You can construct a :class:~django.core.files.File from an existing
Python file object like this::

from django.core.files import File
# Open an existing file using Python's built-in open()
f = open('/path/to/hello.world')
myfile = File(f)

Or you can construct one from a Python string like this::

from django.core.files.base import ContentFile
myfile = ContentFile("hello world")

For more information, see :doc:/topics/files.

.. method:: FieldFile.delete(save=True)

Deletes the file associated with this instance and clears all attributes on
the field. Note: This method will close the file if it happens to be open when
delete() is called.

The optional save argument controls whether or not the model instance is
saved after the file associated with this field has been deleted. Defaults to
True.

Note that when a model is deleted, related files are not deleted. If you need
to cleanup orphaned files, you'll need to handle it yourself (for instance,
with a custom management command that can be run manually or scheduled to run
periodically via e.g. cron).

.. currentmodule:: django.db.models

FilePathField

.. class:: FilePathField(path=None, match=None, recursive=False, max_length=100, **options)

A :class:CharField whose choices are limited to the filenames in a certain
directory on the filesystem. Has three special arguments, of which the first is
required:

.. attribute:: FilePathField.path

Required. The absolute filesystem path to a directory from which this
:class:`FilePathField` should get its choices. Example: ``"/home/images"``.

.. attribute:: FilePathField.match

Optional. A regular expression, as a string, that :class:`FilePathField`
will use to filter filenames. Note that the regex will be applied to the
base filename, not the full path. Example: ``"foo.*\.txt$"``, which will
match a file called ``foo23.txt`` but not ``bar.txt`` or ``foo23.png``.

.. attribute:: FilePathField.recursive

Optional. Either ``True`` or ``False``. Default is ``False``. Specifies
whether all subdirectories of :attr:`~FilePathField.path` should be included

.. attribute:: FilePathField.allow_files

Optional.  Either ``True`` or ``False``.  Default is ``True``.  Specifies
whether files in the specified location should be included.  Either this or
:attr:`~FilePathField.allow_folders` must be ``True``.

.. attribute:: FilePathField.allow_folders

Optional.  Either ``True`` or ``False``.  Default is ``False``.  Specifies
whether folders in the specified location should be included.  Either this
or :attr:`~FilePathField.allow_files` must be ``True``.

Of course, these arguments can be used together.

The one potential gotcha is that :attr:~FilePathField.match applies to the
base filename, not the full path. So, this example::

FilePathField(path="/home/images", match="foo.*", recursive=True)

...will match /home/images/foo.png but not /home/images/foo/bar.png
because the :attr:~FilePathField.match applies to the base filename
(foo.png and bar.png).

:class:FilePathField instances are created in your database as varchar
columns with a default max length of 100 characters. As with other fields, you
can change the maximum length using the :attr:~CharField.max_length argument.

FloatField

.. class:: FloatField(**options)

A floating-point number represented in Python by a float instance.

The default form widget for this field is a :class:~django.forms.NumberInput
when :attr:~django.forms.Field.localize is False or
:class:~django.forms.TextInput otherwise.

.. _floatfield_vs_decimalfield:

.. admonition:: FloatField vs. DecimalField

The :class:`FloatField` class is sometimes mixed up with the
:class:`DecimalField` class. Although they both represent real numbers, they
represent those numbers differently. ``FloatField`` uses Python's ``float``
type internally, while ``DecimalField`` uses Python's ``Decimal`` type. For
information on the difference between the two, see Python's documentation
for the :mod:`decimal` module.

ImageField

.. class:: ImageField(upload_to=None, height_field=None, width_field=None, max_length=100, **options)

Inherits all attributes and methods from :class:FileField, but also
validates that the uploaded object is a valid image.

In addition to the special attributes that are available for :class:FileField,
an :class:ImageField also has height and width attributes.

To facilitate querying on those attributes, :class:ImageField has two extra
optional arguments:

.. attribute:: ImageField.height_field

Name of a model field which will be auto-populated with the height of the
image each time the model instance is saved.

.. attribute:: ImageField.width_field

Name of a model field which will be auto-populated with the width of the
image each time the model instance is saved.

Requires the Pillow_ library.

.. _Pillow: https://pillow.readthedocs.io/en/latest/

:class:ImageField instances are created in your database as varchar
columns with a default max length of 100 characters. As with other fields, you
can change the maximum length using the :attr:~CharField.max_length argument.

The default form widget for this field is a
:class:~django.forms.ClearableFileInput.

IntegerField

.. class:: IntegerField(**options)

An integer. Values from -2147483648 to 2147483647 are safe in all
databases supported by Django. The default form widget for this field is a
:class:~django.forms.NumberInput when :attr:~django.forms.Field.localize
is False or :class:~django.forms.TextInput otherwise.

GenericIPAddressField

.. class:: GenericIPAddressField(protocol='both', unpack_ipv4=False, **options)

An IPv4 or IPv6 address, in string format (e.g. 192.0.2.30 or
2a02:42fe::4). The default form widget for this field is a
:class:~django.forms.TextInput.

The IPv6 address normalization follows :rfc:4291#section-2.2 section 2.2,
including using the IPv4 format suggested in paragraph 3 of that section, like
::ffff:192.0.2.0. For example, 2001:0::0:01 would be normalized to
2001::1, and ::ffff:0a0a:0a0a to ::ffff:10.10.10.10. All characters
are converted to lowercase.

.. attribute:: GenericIPAddressField.protocol

Limits valid inputs to the specified protocol.
Accepted values are ``'both'`` (default), ``'IPv4'``
or ``'IPv6'``. Matching is case insensitive.

.. attribute:: GenericIPAddressField.unpack_ipv4

Unpacks IPv4 mapped addresses like ``::ffff:192.0.2.1``.
If this option is enabled that address would be unpacked to
``192.0.2.1``. Default is disabled. Can only be used
when ``protocol`` is set to ``'both'``.

If you allow for blank values, you have to allow for null values since blank
values are stored as null.

NullBooleanField

.. class:: NullBooleanField(**options)

Like a :class:BooleanField, but allows NULL as one of the options. Use
this instead of a :class:BooleanField with null=True. The default form
widget for this field is a :class:~django.forms.NullBooleanSelect.

PositiveIntegerField

.. class:: PositiveIntegerField(**options)

Like an :class:IntegerField, but must be either positive or zero (0).
Values from 0 to 2147483647 are safe in all databases supported by
Django. The value 0 is accepted for backward compatibility reasons.

PositiveSmallIntegerField

.. class:: PositiveSmallIntegerField(**options)

Like a :class:PositiveIntegerField, but only allows values under a certain
(database-dependent) point. Values from 0 to 32767 are safe in all
databases supported by Django.

SlugField

.. class:: SlugField(max_length=50, **options)

:term:Slug is a newspaper term. A slug is a short label for something,
containing only letters, numbers, underscores or hyphens. They're generally used
in URLs.

Like a CharField, you can specify :attr:~CharField.max_length (read the note
about database portability and :attr:~CharField.max_length in that section,
too). If :attr:~CharField.max_length is not specified, Django will use a
default length of 50.

Implies setting :attr:Field.db_index to True.

It is often useful to automatically prepopulate a SlugField based on the value
of some other value. You can do this automatically in the admin using
:attr:~django.contrib.admin.ModelAdmin.prepopulated_fields.

.. attribute:: SlugField.allow_unicode

If ``True``, the field accepts Unicode letters in addition to ASCII
letters. Defaults to ``False``.

SmallIntegerField

.. class:: SmallIntegerField(**options)

Like an :class:IntegerField, but only allows values under a certain
(database-dependent) point. Values from -32768 to 32767 are safe in all
databases supported by Django.

TextField

.. class:: TextField(**options)

A large text field. The default form widget for this field is a
:class:~django.forms.Textarea.

If you specify a max_length attribute, it will be reflected in the
:class:~django.forms.Textarea widget of the auto-generated form field.
However it is not enforced at the model or database level. Use a
:class:CharField for that.

.. admonition:: MySQL users

If you are using this field with MySQLdb 1.2.1p2 and the ``utf8_bin``
collation (which is *not* the default), there are some issues to be aware
of. Refer to the :ref:`MySQL database notes ` for
details.

TimeField

.. class:: TimeField(auto_now=False, auto_now_add=False, **options)

A time, represented in Python by a datetime.time instance. Accepts the same
auto-population options as :class:DateField.

The default form widget for this field is a :class:~django.forms.TextInput.
The admin adds some JavaScript shortcuts.

URLField

.. class:: URLField(max_length=200, **options)

A :class:CharField for a URL.

The default form widget for this field is a :class:~django.forms.TextInput.

Like all :class:CharField subclasses, :class:URLField takes the optional
:attr:~CharField.max_length argument. If you don't specify
:attr:~CharField.max_length, a default of 200 is used.

UUIDField

.. class:: UUIDField(**options)

A field for storing universally unique identifiers. Uses Python's
:class:~python:uuid.UUID class. When used on PostgreSQL, this stores in a
uuid datatype, otherwise in a char(32).

Universally unique identifiers are a good alternative to :class:AutoField for
:attr:~Field.primary_key. The database will not generate the UUID for you, so
it is recommended to use :attr:~Field.default::

import uuid
from django.db import models

class MyUUIDModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    # other fields

Note that a callable (with the parentheses omitted) is passed to default,
not an instance of UUID.

Relationship fields

.. module:: django.db.models.fields.related
:synopsis: Related field types

.. currentmodule:: django.db.models

Django also defines a set of fields that represent relations.

.. _ref-foreignkey:

ForeignKey

.. class:: ForeignKey(othermodel, on_delete, **options)

A many-to-one relationship. Requires a positional argument: the class to which
the model is related.

.. _recursive-relationships:

To create a recursive relationship -- an object that has a many-to-one
relationship with itself -- use models.ForeignKey('self', on_delete=models.CASCADE).

.. _lazy-relationships:

If you need to create a relationship on a model that has not yet been defined,
you can use the name of the model, rather than the model object itself::

from django.db import models

class Car(models.Model):
    manufacturer = models.ForeignKey(
        'Manufacturer',
        on_delete=models.CASCADE,
    )
    # ...

class Manufacturer(models.Model):
    # ...
    pass

Relationships defined this way on :ref:abstract models are resolved when the model is subclassed as a
concrete model and are not relative to the abstract model's app_label:

.. snippet::
:filename: products/models.py

from django.db import models

class AbstractCar(models.Model):
    manufacturer = models.ForeignKey('Manufacturer', on_delete=models.CASCADE)

    class Meta:
        abstract = True

.. snippet::
:filename: production/models.py

from django.db import models
from products.models import AbstractCar

class Manufacturer(models.Model):
    pass

class Car(AbstractCar):
    pass

# Car.manufacturer will point to `production.Manufacturer` here.

To refer to models defined in another application, you can explicitly specify
a model with the full application label. For example, if the Manufacturer
model above is defined in another application called production, you'd
need to use::

class Car(models.Model):
    manufacturer = models.ForeignKey(
        'production.Manufacturer',
        on_delete=models.CASCADE,
    )

This sort of reference can be useful when resolving circular import
dependencies between two applications.

A database index is automatically created on the ForeignKey. You can
disable this by setting :attr:~Field.db_index to False. You may want to
avoid the overhead of an index if you are creating a foreign key for
consistency rather than joins, or if you will be creating an alternative index
like a partial or multiple column index.

Database Representation
~~~~~~~~~~~~~~~~~~~~~~~

Behind the scenes, Django appends "_id" to the field name to create its
database column name. In the above example, the database table for the Car
model will have a manufacturer_id column. (You can change this explicitly by
specifying :attr:~Field.db_column) However, your code should never have to
deal with the database column name, unless you write custom SQL. You'll always
deal with the field names of your model object.

.. _foreign-key-arguments:

Arguments
~~~~~~~~~

:class:ForeignKey accepts other arguments that define the details of how the
relation works.

.. attribute:: ForeignKey.on_delete

When an object referenced by a :class:`ForeignKey` is deleted, Django will
emulate the behavior of the SQL constraint specified by the
:attr:`on_delete` argument. For example, if you have a nullable
:class:`ForeignKey` and you want it to be set null when the referenced
object is deleted::

    user = models.ForeignKey(
        User,
        models.SET_NULL,
        blank=True,
        null=True,
    )

.. deprecated:: 1.9

    :attr:`~ForeignKey.on_delete` will become a required argument in Django
    2.0. In older versions it defaults to ``CASCADE``.

The possible values for :attr:~ForeignKey.on_delete are found in
:mod:django.db.models:

• .. attribute:: CASCADE

  Cascade deletes. Django emulates the behavior of the SQL constraint ON
  DELETE CASCADE and also deletes the object containing the ForeignKey.

• .. attribute:: PROTECT

  Prevent deletion of the referenced object by raising
  :exc:~django.db.models.ProtectedError, a subclass of
  :exc:django.db.IntegrityError.

• .. attribute:: SET_NULL

  Set the :class:ForeignKey null; this is only possible if
  :attr:~Field.null is True.

• .. attribute:: SET_DEFAULT

  Set the :class:ForeignKey to its default value; a default for the
  :class:ForeignKey must be set.

• .. function:: SET()

  Set the :class:ForeignKey to the value passed to
  :func:~django.db.models.SET(), or if a callable is passed in,
  the result of calling it. In most cases, passing a callable will be
  necessary to avoid executing queries at the time your models.py is
  imported::

    from django.conf import settings
    from django.contrib.auth import get_user_model
    from django.db import models
  
    def get_sentinel_user():
        return get_user_model().objects.get_or_create(username='deleted')[0]
  
    class MyModel(models.Model):
        user = models.ForeignKey(
            settings.AUTH_USER_MODEL,
            on_delete=models.SET(get_sentinel_user),
        )

• .. attribute:: DO_NOTHING

  Take no action. If your database backend enforces referential
  integrity, this will cause an :exc:~django.db.IntegrityError unless
  you manually add an SQL ON DELETE constraint to the database field.

.. attribute:: ForeignKey.limit_choices_to

Sets a limit to the available choices for this field when this field is
rendered using a ``ModelForm`` or the admin (by default, all objects
in the queryset are available to choose). Either a dictionary, a
:class:`~django.db.models.Q` object, or a callable returning a
dictionary or :class:`~django.db.models.Q` object can be used.

For example::

    staff_member = models.ForeignKey(
        User,
        on_delete=models.CASCADE,
        limit_choices_to={'is_staff': True},
    )

causes the corresponding field on the ``ModelForm`` to list only ``Users``
that have ``is_staff=True``. This may be helpful in the Django admin.

The callable form can be helpful, for instance, when used in conjunction
with the Python ``datetime`` module to limit selections by date range. For
example::

    def limit_pub_date_choices():
        return {'pub_date__lte': datetime.date.utcnow()}

    limit_choices_to = limit_pub_date_choices

If ``limit_choices_to`` is or returns a :class:`Q object
`, which is useful for :ref:`complex queries
`, then it will only have an effect on the choices
available in the admin when the field is not listed in
:attr:`~django.contrib.admin.ModelAdmin.raw_id_fields` in the
``ModelAdmin`` for the model.

.. note::

    If a callable is used for ``limit_choices_to``, it will be invoked
    every time a new form is instantiated. It may also be invoked when a
    model is validated, for example by management commands or the admin.
    The admin constructs querysets to validate its form inputs in various
    edge cases multiple times, so there is a possibility your callable may
    be invoked several times.

.. attribute:: ForeignKey.related_name

The name to use for the relation from the related object back to this one.
It's also the default value for :attr:`related_query_name` (the name to use
for the reverse filter name from the target model). See the :ref:`related
objects documentation ` for a full explanation
and example. Note that you must set this value when defining relations on
:ref:`abstract models `; and when you do so
:ref:`some special syntax ` is available.

If you'd prefer Django not to create a backwards relation, set
``related_name`` to ``'+'`` or end it with ``'+'``. For example, this will
ensure that the ``User`` model won't have a backwards relation to this
model::

    user = models.ForeignKey(
        User,
        on_delete=models.CASCADE,
        related_name='+',
    )

.. attribute:: ForeignKey.related_query_name

The name to use for the reverse filter name from the target model. It
defaults to the value of :attr:`related_name` or
:attr:`~django.db.models.Options.default_related_name` if set, otherwise it
defaults to the name of the model::

    # Declare the ForeignKey with related_query_name
    class Tag(models.Model):
        article = models.ForeignKey(
            Article,
            on_delete=models.CASCADE,
            related_name="tags",
            related_query_name="tag",
        )
        name = models.CharField(max_length=255)

    # That's now the name of the reverse filter
    Article.objects.filter(tag__name="important")

Like :attr:`related_name`, ``related_query_name`` supports app label and
class interpolation via :ref:`some special syntax `.

.. attribute:: ForeignKey.to_field

The field on the related object that the relation is to. By default, Django
uses the primary key of the related object.

.. attribute:: ForeignKey.db_constraint

Controls whether or not a constraint should be created in the database for
this foreign key. The default is ``True``, and that's almost certainly what
you want; setting this to ``False`` can be very bad for data integrity.
That said, here are some scenarios where you might want to do this:

* You have legacy data that is not valid.
* You're sharding your database.

If this is set to ``False``, accessing a related object that doesn't exist
will raise its ``DoesNotExist`` exception.

.. attribute:: ForeignKey.swappable

Controls the migration framework's reaction if this :class:`ForeignKey`
is pointing at a swappable model. If it is ``True`` - the default -
then if the :class:`ForeignKey` is pointing at a model which matches
the current value of ``settings.AUTH_USER_MODEL`` (or another swappable
model setting) the relationship will be stored in the migration using
a reference to the setting, not to the model directly.

You only want to override this to be ``False`` if you are sure your
model should always point towards the swapped-in model - for example,
if it is a profile model designed specifically for your custom user model.

Setting it to ``False`` does not mean you can reference a swappable model
even if it is swapped out - ``False`` just means that the migrations made
with this ForeignKey will always reference the exact model you specify
(so it will fail hard if the user tries to run with a User model you don't
support, for example).

If in doubt, leave it to its default of ``True``.

ManyToManyField

.. class:: ManyToManyField(othermodel, **options)

A many-to-many relationship. Requires a positional argument: the class to
which the model is related, which works exactly the same as it does for
:class:ForeignKey, including :ref:recursive and
:ref:lazy relationships.

Related objects can be added, removed, or created with the field's
:class:~django.db.models.fields.related.RelatedManager.

Database Representation
~~~~~~~~~~~~~~~~~~~~~~~

Behind the scenes, Django creates an intermediary join table to represent the
many-to-many relationship. By default, this table name is generated using the
name of the many-to-many field and the name of the table for the model that
contains it. Since some databases don't support table names above a certain
length, these table names will be automatically truncated to 64 characters and a
uniqueness hash will be used. This means you might see table names like
author_books_9cdf4; this is perfectly normal. You can manually provide the
name of the join table using the :attr:~ManyToManyField.db_table option.

.. _manytomany-arguments:

Arguments
~~~~~~~~~

:class:ManyToManyField accepts an extra set of arguments -- all optional --
that control how the relationship functions.

.. attribute:: ManyToManyField.related_name

Same as :attr:`ForeignKey.related_name`.

.. attribute:: ManyToManyField.related_query_name

Same as :attr:`ForeignKey.related_query_name`.

.. attribute:: ManyToManyField.limit_choices_to

Same as :attr:`ForeignKey.limit_choices_to`.

``limit_choices_to`` has no effect when used on a ``ManyToManyField`` with a
custom intermediate table specified using the
:attr:`~ManyToManyField.through` parameter.

.. attribute:: ManyToManyField.symmetrical

Only used in the definition of ManyToManyFields on self. Consider the
following model::

    from django.db import models

    class Person(models.Model):
        friends = models.ManyToManyField("self")

When Django processes this model, it identifies that it has a
:class:`ManyToManyField` on itself, and as a result, it doesn't add a
``person_set`` attribute to the ``Person`` class. Instead, the
:class:`ManyToManyField` is assumed to be symmetrical -- that is, if I am
your friend, then you are my friend.

If you do not want symmetry in many-to-many relationships with ``self``, set
:attr:`~ManyToManyField.symmetrical` to ``False``. This will force Django to
add the descriptor for the reverse relationship, allowing
:class:`ManyToManyField` relationships to be non-symmetrical.

.. attribute:: ManyToManyField.through

Django will automatically generate a table to manage many-to-many
relationships. However, if you want to manually specify the intermediary
table, you can use the :attr:`~ManyToManyField.through` option to specify
the Django model that represents the intermediate table that you want to
use.

The most common use for this option is when you want to associate
:ref:`extra data with a many-to-many relationship
`.

If you don't specify an explicit ``through`` model, there is still an
implicit ``through`` model class you can use to directly access the table
created to hold the association. It has three fields to link the models.

If the source and target models differ, the following fields are
generated:

* ``id``: the primary key of the relation.
* ``_id``: the ``id`` of the model that declares the
  ``ManyToManyField``.
* ``_id``: the ``id`` of the model that the
  ``ManyToManyField`` points to.

If the ``ManyToManyField`` points from and to the same model, the following
fields are generated:

* ``id``: the primary key of the relation.
* ``from__id``: the ``id`` of the instance which points at the
  model (i.e. the source instance).
* ``to__id``: the ``id`` of the instance to which the relationship
  points (i.e. the target model instance).

This class can be used to query associated records for a given model
instance like a normal model.

.. attribute:: ManyToManyField.through_fields

Only used when a custom intermediary model is specified. Django will
normally determine which fields of the intermediary model to use in order
to establish a many-to-many relationship automatically. However,
consider the following models::

    from django.db import models

    class Person(models.Model):
        name = models.CharField(max_length=50)

    class Group(models.Model):
        name = models.CharField(max_length=128)
        members = models.ManyToManyField(
            Person,
            through='Membership',
            through_fields=('group', 'person'),
        )

    class Membership(models.Model):
        group = models.ForeignKey(Group, on_delete=models.CASCADE)
        person = models.ForeignKey(Person, on_delete=models.CASCADE)
        inviter = models.ForeignKey(
            Person,
            on_delete=models.CASCADE,
            related_name="membership_invites",
        )
        invite_reason = models.CharField(max_length=64)

``Membership`` has *two* foreign keys to ``Person`` (``person`` and
``inviter``), which makes the relationship ambiguous and Django can't know
which one to use. In this case, you must explicitly specify which
foreign keys Django should use using ``through_fields``, as in the example
above.

``through_fields`` accepts a 2-tuple ``('field1', 'field2')``, where
``field1`` is the name of the foreign key to the model the
:class:`ManyToManyField` is defined on (``group`` in this case), and
``field2`` the name of the foreign key to the target model (``person``
in this case).

When you have more than one foreign key on an intermediary model to any
(or even both) of the models participating in a many-to-many relationship,
you *must* specify ``through_fields``. This also applies to
:ref:`recursive relationships `
when an intermediary model is used and there are more than two
foreign keys to the model, or you want to explicitly specify which two
Django should use.

Recursive relationships using an intermediary model are always defined as
non-symmetrical -- that is, with :attr:`symmetrical=False `
-- therefore, there is the concept of a "source" and a "target". In that
case ``'field1'`` will be treated as the "source" of the relationship and
``'field2'`` as the "target".

.. attribute:: ManyToManyField.db_table

The name of the table to create for storing the many-to-many data. If this
is not provided, Django will assume a default name based upon the names of:
the table for the model defining the relationship and the name of the field
itself.

.. attribute:: ManyToManyField.db_constraint

Controls whether or not constraints should be created in the database for
the foreign keys in the intermediary table. The default is ``True``, and
that's almost certainly what you want; setting this to ``False`` can be
very bad for data integrity. That said, here are some scenarios where you
might want to do this:

* You have legacy data that is not valid.
* You're sharding your database.

It is an error to pass both ``db_constraint`` and ``through``.

.. attribute:: ManyToManyField.swappable

Controls the migration framework's reaction if this :class:`ManyToManyField`
is pointing at a swappable model. If it is ``True`` - the default -
then if the :class:`ManyToManyField` is pointing at a model which matches
the current value of ``settings.AUTH_USER_MODEL`` (or another swappable
model setting) the relationship will be stored in the migration using
a reference to the setting, not to the model directly.

You only want to override this to be ``False`` if you are sure your
model should always point towards the swapped-in model - for example,
if it is a profile model designed specifically for your custom user model.

If in doubt, leave it to its default of ``True``.

:class:ManyToManyField does not support :attr:~Field.validators.

:attr:~Field.null has no effect since there is no way to require a
relationship at the database level.

OneToOneField

.. class:: OneToOneField(othermodel, on_delete, parent_link=False, **options)

A one-to-one relationship. Conceptually, this is similar to a
:class:ForeignKey with :attr:unique=True , but the
"reverse" side of the relation will directly return a single object.

This is most useful as the primary key of a model which "extends"
another model in some way; :ref:multi-table-inheritance is
implemented by adding an implicit one-to-one relation from the child
model to the parent model, for example.

One positional argument is required: the class to which the model will be
related. This works exactly the same as it does for :class:ForeignKey,
including all the options regarding :ref:recursive
and :ref:lazy relationships.

If you do not specify the :attr:~ForeignKey.related_name argument for
the OneToOneField, Django will use the lower-case name of the current model
as default value.

With the following example::

from django.conf import settings
from django.db import models

class MySpecialUser(models.Model):
    user = models.OneToOneField(
        settings.AUTH_USER_MODEL,
        on_delete=models.CASCADE,
    )
    supervisor = models.OneToOneField(
        settings.AUTH_USER_MODEL,
        on_delete=models.CASCADE,
        related_name='supervisor_of',
    )

your resulting User model will have the following attributes::

>>> user = User.objects.get(pk=1)
>>> hasattr(user, 'myspecialuser')
True
>>> hasattr(user, 'supervisor_of')
True

A DoesNotExist exception is raised when accessing the reverse relationship
if an entry in the related table doesn't exist. For example, if a user doesn't
have a supervisor designated by MySpecialUser::

>>> user.supervisor_of
Traceback (most recent call last):
    ...
DoesNotExist: User matching query does not exist.

.. _onetoone-arguments:

Additionally, OneToOneField accepts all of the extra arguments
accepted by :class:ForeignKey, plus one extra argument:

.. attribute:: OneToOneField.parent_link

When ``True`` and used in a model which inherits from another
:term:`concrete model`, indicates that this field should be used as the
link back to the parent class, rather than the extra
``OneToOneField`` which would normally be implicitly created by
subclassing.

See :doc:One-to-one relationships for usage
examples of OneToOneField.

Field API reference

.. class:: Field

``Field`` is an abstract class that represents a database table column.
Django uses fields to create the database table (:meth:`db_type`), to map
Python types to database (:meth:`get_prep_value`) and vice-versa
(:meth:`from_db_value`).

A field is thus a fundamental piece in different Django APIs, notably,
:class:`models ` and :class:`querysets
`.

In models, a field is instantiated as a class attribute and represents a
particular table column, see :doc:`/topics/db/models`. It has attributes
such as :attr:`null` and :attr:`unique`, and methods that Django uses to
map the field value to database-specific values.

A ``Field`` is a subclass of
:class:`~django.db.models.lookups.RegisterLookupMixin` and thus both
:class:`~django.db.models.Transform` and
:class:`~django.db.models.Lookup` can be registered on it to be used
in ``QuerySet``\s (e.g. ``field_name__exact="foo"``). All :ref:`built-in
lookups ` are registered by default.

All of Django's built-in fields, such as :class:`CharField`, are particular
implementations of ``Field``. If you need a custom field, you can either
subclass any of the built-in fields or write a ``Field`` from scratch. In
either case, see :doc:`/howto/custom-model-fields`.

.. attribute:: description

    A verbose description of the field, e.g. for the
    :mod:`django.contrib.admindocs` application.

    The description can be of the form::

        description = _("String (up to %(max_length)s)")

    where the arguments are interpolated from the field's ``__dict__``.

To map a ``Field`` to a database-specific type, Django exposes several
methods:

.. method:: get_internal_type()

    Returns a string naming this field for backend specific purposes.
    By default, it returns the class name.

    See :ref:`emulating-built-in-field-types` for usage in custom fields.

.. method:: db_type(connection)

    Returns the database column data type for the :class:`Field`, taking
    into account the ``connection``.

    See :ref:`custom-database-types` for usage in custom fields.

.. method:: rel_db_type(connection)

    .. versionadded:: 1.10

    Returns the database column data type for fields such as ``ForeignKey``
    and ``OneToOneField`` that point to the :class:`Field`, taking
    into account the ``connection``.

    See :ref:`custom-database-types` for usage in custom fields.

There are three main situations where Django needs to interact with the
database backend and fields:

* when it queries the database (Python value -> database backend value)
* when it loads data from the database (database backend value -> Python
  value)
* when it saves to the database (Python value -> database backend value)

When querying, :meth:`get_db_prep_value` and :meth:`get_prep_value` are used:

.. method:: get_prep_value(value)

    ``value`` is the current value of the model's attribute, and the method
    should return data in a format that has been prepared for use as a
    parameter in a query.

    See :ref:`converting-python-objects-to-query-values` for usage.

.. method:: get_db_prep_value(value, connection, prepared=False)

    Converts ``value`` to a backend-specific value. By default it returns
    ``value`` if ``prepared=True`` and :meth:`~Field.get_prep_value` if is
    ``False``.

    See :ref:`converting-query-values-to-database-values` for usage.

When loading data, :meth:`from_db_value` is used:

.. method:: from_db_value(value, expression, connection, context)

    Converts a value as returned by the database to a Python object. It is
    the reverse of :meth:`get_prep_value`.

    This method is not used for most built-in fields as the database
    backend already returns the correct Python type, or the backend itself
    does the conversion.

    See :ref:`converting-values-to-python-objects` for usage.

    .. note::

        For performance reasons, ``from_db_value`` is not implemented as a
        no-op on fields which do not require it (all Django fields).
        Consequently you may not call ``super`` in your definition.

When saving, :meth:`pre_save` and :meth:`get_db_prep_save` are used:

.. method:: get_db_prep_save(value, connection)

    Same as the :meth:`get_db_prep_value`, but called when the field value
    must be *saved* to the database. By default returns
    :meth:`get_db_prep_value`.

.. method:: pre_save(model_instance, add)

    Method called prior to :meth:`get_db_prep_save` to prepare the value
    before being saved (e.g. for :attr:`DateField.auto_now`).

    ``model_instance`` is the instance this field belongs to and ``add``
    is whether the instance is being saved to the database for the first
    time.

    It should return the value of the appropriate attribute from
    ``model_instance`` for this field. The attribute name is in
    ``self.attname`` (this is set up by :class:`~django.db.models.Field`).

    See :ref:`preprocessing-values-before-saving` for usage.

Fields often receive their values as a different type, either from
serialization or from forms.

.. method:: to_python(value)

    Converts the value into the correct Python object. It acts as the
    reverse of :meth:`value_to_string`, and is also called in
    :meth:`~django.db.models.Model.clean`.

    See :ref:`converting-values-to-python-objects` for usage.

Besides saving to the database, the field also needs to know how to
serialize its value:

.. method:: value_to_string(obj)

    Converts ``obj`` to a string. Used to serialize the value of the field.

    See :ref:`converting-model-field-to-serialization` for usage.

When using :class:`model forms `, the ``Field``
needs to know which form field it should be represented by:

.. method:: formfield(form_class=None, choices_form_class=None, **kwargs)

    Returns the default :class:`django.forms.Field` of this field for
    :class:`~django.forms.ModelForm`.

    By default, if both ``form_class`` and ``choices_form_class`` are
    ``None``, it uses :class:`~django.forms.CharField`. If the field has
    :attr:`~django.db.models.Field.choices` and ``choices_form_class``
    isn't specified, it uses :class:`~django.forms.TypedChoiceField`.

    See :ref:`specifying-form-field-for-model-field` for usage.

.. method:: deconstruct()

    Returns a 4-tuple with enough information to recreate the field:

    1. The name of the field on the model.
    2. The import path of the field (e.g. ``"django.db.models.IntegerField"``).
       This should be the most portable version, so less specific may be better.
    3. A list of positional arguments.
    4. A dict of keyword arguments.

    This method must be added to fields prior to 1.7 to migrate its data
    using :doc:`/topics/migrations`.

.. _model-field-attributes:

=========================
Field attribute reference
=========================

Every Field instance contains several attributes that allow
introspecting its behavior. Use these attributes instead of isinstance
checks when you need to write code that depends on a field's functionality.
These attributes can be used together with the :ref:Model._meta API to narrow down a search for specific field types.
Custom model fields should implement these flags.

Attributes for fields

.. attribute:: Field.auto_created

 Boolean flag that indicates if the field was automatically created, such
 as the ``OneToOneField`` used by model inheritance.

.. attribute:: Field.concrete

Boolean flag that indicates if the field has a database column associated
with it.

.. attribute:: Field.hidden

Boolean flag that indicates if a field is used to back another non-hidden
field's functionality (e.g. the ``content_type`` and ``object_id`` fields
that make up a ``GenericForeignKey``). The ``hidden`` flag is used to
distinguish what constitutes the public subset of fields on the model from
all the fields on the model.

.. note::

    :meth:`Options.get_fields()
    `
    excludes hidden fields by default. Pass in ``include_hidden=True`` to
    return hidden fields in the results.

.. attribute:: Field.is_relation

Boolean flag that indicates if a field contains references to one or
more other models for its functionality (e.g. ``ForeignKey``,
``ManyToManyField``, ``OneToOneField``, etc.).

.. attribute:: Field.model

Returns the model on which the field is defined. If a field is defined on
a superclass of a model, ``model`` will refer to the superclass, not the
class of the instance.

Attributes for fields with relations

These attributes are used to query for the cardinality and other details of a
relation. These attribute are present on all fields; however, they will only
have boolean values (rather than None) if the field is a relation type
(:attr:Field.is_relation=True ).

.. attribute:: Field.many_to_many

Boolean flag that is ``True`` if the field has a many-to-many relation;
``False`` otherwise. The only field included with Django where this is
``True`` is ``ManyToManyField``.

.. attribute:: Field.many_to_one

Boolean flag that is ``True`` if the field has a many-to-one relation, such
as a ``ForeignKey``; ``False`` otherwise.

.. attribute:: Field.one_to_many

Boolean flag that is ``True`` if the field has a one-to-many relation, such
as a ``GenericRelation`` or the reverse of a ``ForeignKey``; ``False``
otherwise.

.. attribute:: Field.one_to_one

Boolean flag that is ``True`` if the field has a one-to-one relation, such
as a ``OneToOneField``; ``False`` otherwise.

.. attribute:: Field.related_model

Points to the model the field relates to. For example, ``Author`` in
``ForeignKey(Author, on_delete=models.CASCADE)``. If a field has a generic
relation (such as a ``GenericForeignKey`` or a ``GenericRelation``) then
``related_model`` will be ``None``.

转载于:https://www.cnblogs.com/xiangnan/p/5880349.html