Difference between has_one and belongs_to in Rails? [duplicate]

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?

Stilbestrol

The code above is incorrect, should be has_one :cell and belongs_to :person there should not be a space between the colon and the following word. – Armitage

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.

Masquerade

Nice answer. Seeing your answer I realize I read half the question. I'm sorry, but glad that you jumped in. + 10 – Meingolda

I've had to look this up a billion times. I wish they had thought through the naming a little better, to make it more clear which one goes where. – Masquerade

Great, now I know the two wrong answers. Would be better to show the "correct" way to show it. Just sayin'. – Hence

I always think of it in terms of Toy Story. Andy 'has_one' Woody, Woody 'belongs_to' andy. Where is the foreign key? On Woody's sole. – Zymogen

Thinking in terms of Toy Story really helps, so Woody has an 'andy_id' printed on his sole :) – Teaser

That's a cool mnemonic, but thought I'd share my mathematical method of remembering. has_one is like has_many, and has_many implies the key is on the other table since there is fixed number of columns defined on a SQL table. – Ravenous

@Mike that's so adorable. You're a delight. – Dash

Answer 2

If you add "belongs_to" then you got a bidirectional association. That means you can get a person from the cell and a cell from the person.

There's no real difference, both approaches (with and without "belongs_to") use the same database schema (a person_id field in the cells database table).

To summarize: Do not add "belongs_to" unless you need bidirectional associations between models.

Meingolda

The best line I have read today: "If you add "belongs_to" then you got a bidirectional association. That means you can get a person from the cell and a cell from the person." +1 – Aglet

Answer 3

Using both allows you to get info from both Person and Cell models.

@cell.person.whatever_info and @person.cell.whatever_info.

Lido