Which One Goes Where?

One of the most confusing database relationship questions for me was always when to use has_one vs belongs_to. The relationship is a simple one-to-one relationship, and if we think about it in terms of the models, it makes a lot of sense. For example, let's say we have a Dog model and an Owner model. Immediately we can say the Owner has_one Dog and the Dog belongs_to Owner. In this case, the objects are familiar to us from our everyday life and it is easy to figure out. When the objects are more abstract like Foo and Bar it is much more difficult to determine. Let's hold that thought for a moment.

The trickiest part of this whole ordeal for me was always figuring out which model object has the foreign_key on it, the id of the other model with which it is associated. This foreign_key is actually the only difference between the has_one and belongs_to semantic naming in a one-to-one relationship. The question is, does it go on the model that belongs_to the other or the one that has_one of the first prior? Let's go back to my original example with the Owner and the Dog. In this case, the Owner has_one Dog. In real life, most dogs are required to wear collars with some identification, typically the owner's name and address, in case the dog gets lost. Did you catch that? The dog has the owner's id on it, or relating it back to our database model, the foreign_key! Since Dog belongs_to Owner, the foreign_key will be on the Dog model, meaning the object that belongs_to the other will always have the foreign_key that makes the association. This example makes it easy to remember where the foreign_key belongs, but we still do not know how to determine which object belongs to the other in a more abstract case.

Going back to the previously mentioned example with models, Foo and Bar, let's make some assumptions. Let's assume that Foo has_one Bar. For you Rails users, the Rails associations would look as follows.

# app/models/foo.rb
class Foo < ActiveRecord::Base
  has_one :bar
end

# app/models/bar.rb
class Bar < ActiveRecord::Base
  belongs_to :foo
end

If we want to find the Bar that belongs to a given Foo, the SQL would look something like the following.

-- "has_one" query
-- Find the Bar that belongs to a given Foo
-- assume an intance foo of Foo already exists
SELECT *
FROM Bar
WHERE foo_id = foo.id

# Rails Active Record equivalent will call the above SQL behind the scenes
foo = Foo.first
foo.bar

To do the inverse, get a Foo for a given Bar, the SQL would look something like the following:

-- "belongs_to" query
-- Find the Foo that has a given Bar
-- assume an instance bar of Bar already exists
SELECT *
FROM Foo
WHERE id = bar.foo_id

# Rails Active Record equivalent will call the above SQL behind the scenes
bar = Bar.first
bar.foo

Hopefully now you know where to place the foreign_key (think of our Owner has_one Dog example) as well as the underlying reasons that this makes sense. You should also have a good sense of the SQL required to get the association in both directions and understand where to put the foreign_key on models that are more abstract.