验证多态关联中一个或多个模型实例的限制

Question

I have a model Photos with polymorphic associations that is supposed to hold the photos used by other models let say Post and Profile.

I'm trying to validate that:

1. Each instance of Post could only be associated to only one instance of Photo
2. Each instance of Profile could be associated to one or more instances of Photo.

I'm trying to use the following validation code in my Photo model without success

 class Photo < ApplicationRecord
  belongs_to :photoable, polymorphic: true
  validates_uniqueness_of :photoable_id, scope: :photoable_type
end

I was assuming the scope in this validation would instruct Rails to reject repeating ids only for each :photoable_type but what really happens is that Rails rejects any repeating id regardless if the type is set to 'Profile' or 'Post' (eg I can only have one instance of Photo with photoable_id == 1 whilst I wish I could have two instances with same photoable_id as long as one has photoable_type == 'Profile' and the other a photoable_type == 'Post')

Could you help explaining how to achieve this and why scope won't work like that? It would have been the same outcome to have left 'scope: :photoable_type' out of the validation code.

The code in the other 2 models below:

Post model

class Post < ApplicationRecord
  has_one :photo, as: :photoable
end

Profile model

class Profile < ApplicationRecord
  has_many :photos, as: :photoable
end

Answer 1

I used the code suggested in the following post to solve the issue. Validate scoped uniqueness in polymorphic association models.

In this case a working solution would be:

  validates_uniqueness_of : photoable_id, scope: [: photoable_type, :photoable_id]