Difference between has_one and belongs_to in Rails?

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.

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.