belongs_to vs HasMany
Ruby on Rails belongs_to
belongs_to complements a has_many or has_one association.
In general, the Foo model belongs_to :bar if the foo table has a bar_id foreign key column.
Examples of this:
- Companies database, where an account belongs to a company.
- Forum system, where a topic belongs to a forum, which belongs to a category.
- a gallery system where a thumbnail belongs to a picture.
Example Forum
See ForumExample.
Notes
When using :counter_cache => true, be sure the default value on your model_count column defaults to something that can be incremented. NULL + 1, for instance, equals NULL so the counter SQL won’t work.
You’d think this feature would make parent.child.count or parent.child.length use this cache, instead of issuing a new COUNT query. You’d be wrong.
See the API for more information about :counter_cache
Another way to think of this is to say “the foreign key in table abc belongs_to table def”.
Gotchas
Watch your plural versus singular! If you’re going between has_many and belongs_to, it’s easy to get mixed up with plurals and singulars. belongs_to definitely uses singular parameters, and will not print very convenient errors if not put correctly.
If you are using belongs_to across namespaces, you will definitely need to use the :class_name and :foreign_key modifiers to belongs_to.
Say you are saying your Model belongs_to MySpace::OtherModel
class Model
belongs_to :other_model # will NOT work
# The following WILL work
belongs_to :other_model,
:class_name => "MySpace::OtherModel",
:foreign_key => "other_model_id"
end