问题
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?
回答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.
回答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.
回答3:
Using both allows you to get info from both Person and Cell models.
@cell.person.whatever_info and @person.cell.whatever_info.
来源：https://stackoverflow.com/questions/861144/difference-between-has-one-and-belongs-to-in-rails