Differentiate between a has_one and belongs_to association in Ruby?

Question

what is the main differences

Answer 1

belongs_to will place the foreign key in the declaring model whereas has_one will place it in the other model.

To sum things up: use belongs_to when you want the foreign key in the declaring model, use has_one if you want it on the other model.

But anyway, you will rarely see a belongs_to or a has_one used alone. Most of the time it will be has_many with belongs_to for a one-to-many association and has_one with belongs_to for a one-to-one association.