Question

Consider the two models:

1. User => (id: integer, name: string, created_at: datetime, updated_at: datetime, department_id: integer)
2. Department => (id: integer, name: string, created_at: datetime, updated_at: datetime)

Now, there is a obvious relationship between these two tables. it is

• Each User has one department
• A Department belongs to many users i.e there are multiple users in a single department

so, I chose epress the same as following as A belongs_to B

class User < ActiveRecord::Base
  has_one :department
end
class Department < ActiveRecord::Base
  belongs_to :user
end

But as you may know doesn’t work & throws the following error:

> @user = User.find(1)
> @user.department.name
  Department Load (1.0ms)  SELECT "departments".* FROM "departments" WHERE "departments"."user_id" = 1 LIMIT 1
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column departments.user_id does not exist

After lot of hit & trial. I stumbled on the correct way to define this, which is just other way around i.e B belongs to A

class Department < ActiveRecord::Base
  has_many :users
end
class User < ActiveRecord::Base
  belongs_to :department
end
> @user = User.find(1)
> @user.department.name
Department Load (1.0ms)  SELECT "departments".* FROM "departments" WHERE "departments"."id" = 1 LIMIT 1
 => "HR"

Now, this is exact opposite of the way my brain thinks about these associations. So, I am little confused, so if anyone could explain what is going on?

Why does B belongs to A & NOT A belongs to B?

Answer 1

The :has_one relation is a little bit strange at the first look. Consider a user and an account, every account is connected to (belongs to) just one user, from the other side a user has_one account, just one, his account. An account is something that the user owns.

In these cases you use the has_one relation on the User model and Rails will search for a user_id column in the accounts table (see the has_one documentation in the Rails Guide).

In your specific case I think is perfecty right to say “a department has many users” and “a user belongs to a department”.