async-mongo: motor

.. currentmodule:: motor.motor_asyncio

Tutorial: Using Motor With :mod:asyncio

.. These setups are redundant because I can't figure out how to make doctest
run a common setup before the setup for the two groups. A "testsetup:: *"
is the obvious answer, but it's run after group-specific setup.

.. testsetup:: before-inserting-2000-docs

import pymongo
import motor.motor_asyncio
import asyncio
from asyncio import coroutine
db = motor.motor_asyncio.AsyncIOMotorClient().test_database

.. testsetup:: after-inserting-2000-docs

import pymongo
import motor.motor_asyncio
import asyncio
from asyncio import coroutine
db = motor.motor_asyncio.AsyncIOMotorClient().test_database
pymongo.MongoClient().test_database.test_collection.insert_many(
[{'i': i} for i in range(2000)])

.. testcleanup:: *

import pymongo
pymongo.MongoClient().test_database.test_collection.delete_many({})

A guide to using MongoDB and asyncio with Motor.

.. contents::

Tutorial Prerequisites

You can learn about MongoDB with the MongoDB Tutorial_ before you learn Motor.

Using Python 3.4 or later, do::

$ python3 -m pip install motor

This tutorial assumes that a MongoDB instance is running on the
default host and port. Assuming you have downloaded and installed _ MongoDB, you
can start it like so:

.. code-block:: bash

$ mongod

.. _pip: http://www.pip-installer.org/en/latest/installing.html

.. _MongoDB Tutorial: http://docs.mongodb.org/manual/tutorial/getting-started/

Object Hierarchy

Motor, like PyMongo, represents data with a 4-level object hierarchy:

• :class:~motor.motor_asyncio.AsyncIOMotorClient
  represents a mongod process, or a cluster of them. You explicitly create one
  of these client objects, connect it to a running mongod or mongods, and
  use it for the lifetime of your application.
• :class:~motor.motor_asyncio.AsyncIOMotorDatabase: Each mongod has a set of databases (distinct
  sets of data files on disk). You can get a reference to a database from a
  client.
• :class:~motor.motor_asyncio.AsyncIOMotorCollection: A database has a set of collections, which
  contain documents; you get a reference to a collection from a database.
• :class:~motor.motor_asyncio.AsyncIOMotorCursor: Executing :meth:~motor.motor_asyncio.AsyncIOMotorCollection.find on
  an :class:~motor.motor_asyncio.AsyncIOMotorCollection gets an :class:~motor.motor_asyncio.AsyncIOMotorCursor, which
  represents the set of documents matching a query.

Creating a Client

You typically create a single instance of :class:~motor.motor_asyncio.AsyncIOMotorClient at the time your
application starts up.

.. doctest:: before-inserting-2000-docs

import motor.motor_asyncio
client = motor.motor_asyncio.AsyncIOMotorClient()

This connects to a mongod listening on the default host and port. You can
specify the host and port like:

.. doctest:: before-inserting-2000-docs

client = motor.motor_asyncio.AsyncIOMotorClient('localhost', 27017)

Motor also supports connection URIs_:

.. doctest:: before-inserting-2000-docs

client = motor.motor_asyncio.AsyncIOMotorClient('mongodb://localhost:27017')

Connect to a replica set like:

client = motor.motor_asyncio.AsyncIOMotorClient('mongodb://host1,host2/?replicaSet=my-replicaset-name')

.. _connection URIs: http://docs.mongodb.org/manual/reference/connection-string/

Getting a Database

A single instance of MongoDB can support multiple independent
databases _.
From an open client, you can get a reference to a particular database with
dot-notation or bracket-notation:

.. doctest:: before-inserting-2000-docs

db = client.test_database
db = client['test_database']

Creating a reference to a database does no I/O and does not require an
await expression.

Getting a Collection

A collection _
is a group of documents stored in MongoDB, and can be thought of as roughly
the equivalent of a table in a relational database. Getting a
collection in Motor works the same as getting a database:

.. doctest:: before-inserting-2000-docs

collection = db.test_collection
collection = db['test_collection']

Just like getting a reference to a database, getting a reference to a
collection does no I/O and doesn't require an await expression.

Inserting a Document

As in PyMongo, Motor represents MongoDB documents with Python dictionaries. To
store a document in MongoDB, call :meth:~AsyncIOMotorCollection.insert_one in an
await expression:

.. doctest:: before-inserting-2000-docs

async def do_insert():
... document = {'key': 'value'}
... result = await db.test_collection.insert_one(document)
... print('result %s' % repr(result.inserted_id))
...

loop = asyncio.get_event_loop()
loop.run_until_complete(do_insert())
result ObjectId('...')

.. mongodoc:: insert

.. doctest:: before-inserting-2000-docs
:hide:

Clean up from previous insert

pymongo.MongoClient().test_database.test_collection.delete_many({})

Using native coroutines

Starting in Python 3.5, you can define a native coroutine_ with async def
instead of the coroutine decorator. Within a native coroutine, wait
for an async operation with await instead of yield:

.. doctest:: before-inserting-2000-docs

async def do_insert():
... for i in range(2000):
... result = await db.test_collection.insert_one({'i': i})
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_insert())

Within a native coroutine, the syntax to use Motor with Tornado or asyncio
is often identical.

.. _native coroutine: https://www.python.org/dev/peps/pep-0492/

Getting a Single Document With find_one

Use :meth:~motor.motor_asyncio.AsyncIOMotorCollection.find_one to get the first document that
matches a query. For example, to get a document where the value for key "i" is
less than 1:

.. doctest:: after-inserting-2000-docs

async def do_find_one():
... document = await db.test_collection.find_one({'i': {'$lt': 1}})
... pprint.pprint(document)
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_find_one())
{'_id': ObjectId('...'), 'i': 0}

The result is a dictionary matching the one that we inserted previously.

.. note:: The returned document contains an "_id", which was
automatically added on insert.

.. mongodoc:: find

Querying for More Than One Document

Use :meth:~motor.motor_asyncio.AsyncIOMotorCollection.find to query for a set of documents.
:meth:~motor.motor_asyncio.AsyncIOMotorCollection.find does no I/O and does not require an await
expression. It merely creates an :class:~motor.motor_asyncio.AsyncIOMotorCursor instance. The query is
actually executed on the server when you call :meth:~motor.motor_asyncio.AsyncIOMotorCursor.to_list
or execute an async for loop.

To find all documents with "i" less than 5:

.. doctest:: after-inserting-2000-docs

async def do_find():
... cursor = db.test_collection.find({'i': {'$lt': 5}}).sort('i')
... for document in await cursor.to_list(length=100):
... pprint.pprint(document)
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_find())
{'_id': ObjectId('...'), 'i': 0}
{'_id': ObjectId('...'), 'i': 1}
{'_id': ObjectId('...'), 'i': 2}
{'_id': ObjectId('...'), 'i': 3}
{'_id': ObjectId('...'), 'i': 4}

A length argument is required when you call to_list to prevent Motor
from buffering an unlimited number of documents.

async for

You can handle one document at a time in an ``async for`` loop:

.. doctest:: after-inserting-2000-docs

  >>> async def do_find():
  ...     c = db.test_collection
  ...     async for document in c.find({'i': {'$lt': 2}}):
  ...         pprint.pprint(document)
  ...
  >>> loop = asyncio.get_event_loop()
  >>> loop.run_until_complete(do_find())
  {'_id': ObjectId('...'), 'i': 0}
  {'_id': ObjectId('...'), 'i': 1}

You can apply a sort, limit, or skip to a query before you begin iterating:

.. doctest:: after-inserting-2000-docs

  >>> async def do_find():
  ...     cursor = db.test_collection.find({'i': {'$lt': 5}})
  ...     # Modify the query before iterating
  ...     cursor.sort('i', -1).limit(2).skip(2)
  ...     async for document in cursor:
  ...         pprint.pprint(document)
  ...
  >>> loop = asyncio.get_event_loop()
  >>> loop.run_until_complete(do_find())
  {'_id': ObjectId('...'), 'i': 2}
  {'_id': ObjectId('...'), 'i': 1}

The cursor does not actually retrieve each document from the server
individually; it gets documents efficiently in `large batches`_.

.. _`large batches`: https://docs.mongodb.com/manual/tutorial/iterate-a-cursor/#cursor-batches

Iteration in Python 3.3 and 3.4

In Python versions without async for, handle one document at a time with
:attr:~motor.motor_asyncio.AsyncIOMotorCursor.fetch_next
and :meth:~motor.motor_asyncio.AsyncIOMotorCursor.next_object:

.. doctest:: after-inserting-2000-docs

@coroutine
... def do_find():
... cursor = db.test_collection.find({'i': {'$lt': 5}})
... while (yield from cursor.fetch_next):
... document = cursor.next_object()
... pprint.pprint(document)
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_find())
{'_id': ObjectId('...'), 'i': 0}
{'_id': ObjectId('...'), 'i': 1}
async def do_find():
... cursor = db.test_collection.find({'i': {'$lt': 5}})
... # Modify the query before iterating
... cursor.sort('i', -1).limit(2).skip(2)
... async for document in cursor:
... pprint.pprint(document)
...
>>> loop = asyncio.get_event_loop()
>>> loop.run_until_complete(do_find())
{'_id': ObjectId('...'), 'i': 2}
{'id': ObjectId('...'), 'i': 1}

The cursor does not actually retrieve each document from the server
individually; it gets documents efficiently in large batches.

.. _large batches: https://docs.mongodb.com/manual/tutorial/iterate-a-cursor/#cursor-batches

Iteration in Python 3.3 and 3.4
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In Python versions without async for, handle one document at a time with
:attr:~motor.motor_asyncio.AsyncIOMotorCursor.fetch_next
and :meth:~motor.motor_asyncio.AsyncIOMotorCursor.next_object:

.. doctest:: after-inserting-2000-docs

>>> @coroutine
... def do_find():
... cursor = db.test_collection.find({'i': {'$lt': 5}})
... while (yield from cursor.fetch_next):
... document = cursor.next_object()
... pprint.pprint(document)
...
>>> loop = asyncio.get_event_loop()
>>> loop.run_until_complete(do_find())
{'_id': ObjectId('...'), 'i': 0}
{'_id': ObjectId('...'), 'i': 1}
{'_id': ObjectId('...'), 'i': 2}
{'_id': ObjectId('...'), 'i': 3}
{'_id': ObjectId('...'), 'i': 4}

Counting Documents

Use :meth:~motor.motor_asyncio.AsyncIOMotorCursor.count to determine the number of documents in
a collection, or the number of documents that match a query:

.. doctest:: after-inserting-2000-docs

async def do_count():
... n = await db.test_collection.find().count()
... print('%s documents in collection' % n)
... n = await db.test_collection.find({'i': {'$gt': 1000}}).count()
... print('%s documents where i > 1000' % n)
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_count())
2000 documents in collection
999 documents where i > 1000

:meth:~motor.motor_asyncio.AsyncIOMotorCursor.count uses the count command internally; we'll
cover commands_ below.

.. seealso:: Count command _

Updating Documents

:meth:~motor.motor_asyncio.AsyncIOMotorCollection.replace_one changes a document. It requires two
parameters: a query that specifies which document to replace, and a
replacement document. The query follows the same syntax as for :meth:find or
:meth:find_one. To replace a document:

.. doctest:: after-inserting-2000-docs

async def do_replace():
... coll = db.test_collection
... old_document = await coll.find_one({'i': 50})
... print('found document: %s' % pprint.pformat(old_document))
... _id = old_document['_id']
... result = await coll.replace_one({'_id': _id}, {'key': 'value'})
... print('replaced %s document' % result.modified_count)
... new_document = await coll.find_one({'_id': id})
... print('document is now %s' % pprint.pformat(new_document))
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_replace())
found document: {'id': ObjectId('...'), 'i': 50}
replaced 1 document
loop.run_until_complete(do_count())
2000 documents in collection
999 documents where i > 1000

:meth:~motor.motor_asyncio.AsyncIOMotorCursor.count uses the count command internally; we'll
cover commands below.

.. seealso:: Count command

Updating Documents
------------------

:meth:~motor.motor_asyncio.AsyncIOMotorCollection.replace_one changes a document. It requires two
parameters: a query that specifies which document to replace, and a
replacement document. The query follows the same syntax as for :meth:find or
:meth:find_one. To replace a document:

.. doctest:: after-inserting-2000-docs

>>> async def do_replace():
... coll = db.test_collection
... old_document = await coll.find_one({'i': 50})
... print('found document: %s' % pprint.pformat(old_document))
... _id = old_document['_id']
... result = await coll.replace_one({'_id': _id}, {'key': 'value'})
... print('replaced %s document' % result.modified_count)
... new_document = await coll.find_one({'_id': _id})
... print('document is now %s' % pprint.pformat(new_document))
...
>>> loop = asyncio.get_event_loop()
>>> loop.run_until_complete(do_replace())
found document: {'_id': ObjectId('...'), 'i': 50}
replaced 1 document
document is now {'_id': ObjectId('...'), 'key': 'value'}

You can see that :meth:replace_one replaced everything in the old document
except its _id with the new document.

Use :meth:~motor.motor_asyncio.AsyncIOMotorCollection.update_one with MongoDB's modifier operators to
update part of a document and leave the
rest intact. We'll find the document whose "i" is 51 and use the $set
operator to set "key" to "value":

.. doctest:: after-inserting-2000-docs

async def do_update():
... coll = db.test_collection
... result = await coll.update_one({'i': 51}, {'$set': {'key': 'value'}})
... print('updated %s document' % result.modified_count)
... new_document = await coll.find_one({'i': 51})
... print('document is now %s' % pprint.pformat(new_document))
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_update())
updated 1 document
document is now {'_id': ObjectId('...'), 'i': 51, 'key': 'value'}

"key" is set to "value" and "i" is still 51.

:meth:update_one only affects the first document it finds, you can
update all of them with :meth:update_many::

await coll.update_many({'i': {'$gt': 100}},
                       {'$set': {'key': 'value'}})

.. mongodoc:: update

Deleting Documents

:meth:~motor.motor_asyncio.AsyncIOMotorCollection.delete_many takes a query with the same syntax as
:meth:~motor.motor_asyncio.AsyncIOMotorCollection.find.
:meth:delete_many immediately removes all matching documents.

.. doctest:: after-inserting-2000-docs

async def do_delete_many():
... coll = db.test_collection
... n = await coll.count()
... print('%s documents before calling delete_many()' % n)
... result = await db.test_collection.delete_many({'i': {'$gte': 1000}})
... print('%s documents after' % (await coll.count()))
...
loop = asyncio.get_event_loop()
loop.run_until_complete(do_delete_many())
2000 documents before calling delete_many()
1000 documents after

.. mongodoc:: remove

Commands

Besides the "CRUD" operations--insert, update, delete, and find--all other
operations on MongoDB are commands. Run them using
the :meth:~motor.motor_asyncio.AsyncIOMotorDatabase.command method on :class:~motor.motor_asyncio.AsyncIOMotorDatabase:

.. doctest:: after-inserting-2000-docs

from bson import SON
async def use_count_command():
... response = await db.command(SON([("count", "test_collection")]))
... print('response: %s' % pprint.pformat(response))
...
loop = asyncio.get_event_loop()
loop.run_until_complete(use_count_command())
response: {'n': 1000, 'ok': 1.0...}

Since the order of command parameters matters, don't use a Python dict to pass
the command's parameters. Instead, make a habit of using :class:bson.SON,
from the bson module included with PyMongo::

await db.command(SON([("distinct", "test_collection"), ("key", "my_key"]))

Many commands have special helper methods, such as
:meth:~motor.motor_asyncio.AsyncIOMotorDatabase.create_collection or
:meth:~motor.motor_asyncio.AsyncIOMotorCollection.aggregate, but these are just conveniences atop
the basic :meth:command method.

.. mongodoc:: commands

.. _example-web-application-aiohttp:

A Web Application With aiohttp_

Let us create a web application using aiohttp_, a popular HTTP package for
asyncio. Install it with::

python3 -m pip install aiohttp

We are going to make a trivial web site with two pages served from MongoDB.
To begin:

.. literalinclude:: examples/aiohttp_example.py
:language: python3
:start-after: setup-start
:end-before: setup-end

The AsyncIOMotorClient constructor does not actually connect to MongoDB.
The client connects on demand, when you attempt the first operation.
We create it and assign the "test" database's handle to db.

The setup_db coroutine drops the "pages" collection (plainly, this code is
for demonstration purposes), then inserts two documents. Each document's page
name is its unique id, and the "body" field is a simple HTML page. Finally,
setup_db returns the database handle.

We'll use the setup_db coroutine soon. First, we need a request handler
that serves pages from the data we stored in MongoDB.

.. literalinclude:: examples/aiohttp_example.py
:language: python3
:start-after: handler-start
:end-before: handler-end

We start the server by running setup_db and passing the database handle
to an :class:aiohttp.web.Application:

.. literalinclude:: examples/aiohttp_example.py
:language: python3
:start-after: main-start
:end-before: main-end

Note that it is a common mistake to create a new client object for every
request; this comes at a dire performance cost. Create the client
when your application starts and reuse that one client for the lifetime
of the process. You can maintain the client by storing a database handle
from the client on your application object, as shown in this example.

Visit localhost:8080/pages/page-one and the server responds "Hello!".
At localhost:8080/pages/page-two it responds "Goodbye." At other URLs it
returns a 404.

The complete code is in the Motor repository in examples/aiohttp_example.py.

.. _aiohttp: https://aiohttp.readthedocs.io/

See also the :doc:examples/aiohttp_gridfs_example.

Further Reading

The handful of classes and methods introduced here are sufficient for daily
tasks. The API documentation for :class:~motor.motor_asyncio.AsyncIOMotorClient, :class:~motor.motor_asyncio.AsyncIOMotorDatabase,
:class:~motor.motor_asyncio.AsyncIOMotorCollection, and :class:~motor.motor_asyncio.AsyncIOMotorCursor provides a
reference to Motor's complete feature set.

Learning to use the MongoDB driver is just the beginning, of course. For
in-depth instruction in MongoDB itself, see The MongoDB Manual_.