Dumb question

Folks
  Can a model just have a belongs_to only? or do you have to have a
belongs_to and a has_many or has_one on the other model?

Of course you can.

http://guides.rubyonrails.org/association_basics.html#the-belongs_to-association

What belongs_to and their counterparts does is create a method to access an existing relationship, the actual relationship is made in the tables themselves by adding model_id columns, for instance:

class User < ActiveRecord::Base

has_one :address

end

class Address < ActiveRecord::Base

belongs_to :user

end

Shows the address whose addresses.user_id = users.id

User.last.address

Shows the user whose users.id = addresses.user_id

Address.last.user

If you don’t need to access the relationship in the Address model just delete belongs_to :user:

class Address < ActiveRecord::Base

end

User.last.address # Shows address

Address.last.user # NoMethodError: undefined method `user’