Difference between has_one and belongs_to in Rails?

Question

I am trying to understand has_one relationship in RoR.

Let's say I have two models - Person and Cell:

class Person < ActiveRecord::Base
  has_one :cell
end

class Cell < ActiveRecord::Base
  belongs_to :person
end

Can I just use has_one :person instead of belongs_to :person in Cell model?

Isn't it the same?

Answer 1

No, they are not interchangable, and there are some real differences.

belongs_to means that the foreign key is in the table for this class. So belongs_to can ONLY go in the class that holds the foreign key.

has_one means that there is a foreign key in another table that references this class. So has_one can ONLY go in a class that is referenced by a column in another table.

So this is wrong:

class Person < ActiveRecord::Base
  has_one :cell # the cell table has a person_id
end

class Cell < ActiveRecord::Base
  has_one :person # the person table has a cell_id
end

And this is also wrong:

class Person < ActiveRecord::Base
  belongs_to :cell # the person table has a cell_id
end

class Cell < ActiveRecord::Base
  belongs_to :person # the cell table has a person_id
end

The correct way is (if Cell contains person_id field):

class Person < ActiveRecord::Base
  has_one :cell # the person table does not have 'joining' info
end

class Cell < ActiveRecord::Base
  belongs_to :person # the cell table has a person_id
end

For a two-way association, you need one of each, and they have to go in the right class. Even for a one-way association, it matters which one you use.