What is the best way to validate a foreign key association in Rails belongs to current user

Question

As an example, say I have a credit card that belongs to a user with an associated billing address.

One solution is that I assign the relationship in the create/update block. However, this really requires me to handle the errors such as blank id or non-existing record. Was hoping there was a better solution that i have overlooked.

I am interested in making sure that an address that belongs to another user is not assigned to the current user.

So, code for the create solution:

credit_card = current_user.credit_cards.create!(credit_card_params) do |credit_card|
    credit_card.address  = User.addresses.find(params['credit_card']['address_id'])
end

I haven't added the error handling yet and could push that to a class method in the User object and then call User.set_address('address_id') which also handles validation. Is there a better pattern for doing this?

Answer 1

If we look at it from the credit card point of view, you just want to make sure that the address_id is unique, meaning the address_id can't be used twice on two different credit cards.

class CreditCard < ActiveRecord::Base
  validates :address_id, uniqueness: true
end

If a new credit card is initialized with an address_id that belongs to another user the credit card becomes invalid.

Update: by adding a scope to the uniqueness validation, you allow duplication for a smaller set, which in your case would be the user_id ( if 2 creditcards have the same address, and the same user then that's fine )

class CreditCard < ActiveRecord::Base
  validates :address_id, uniqueness: { scope: user_id }
end