Limiting an object to one ownership ID

Question

I have a Flag model that has the possibility of belonging to a Topic, Post, or Review. With that being said the Flag model has all three of their corresponding IDs.

class Flag < ActiveRecord::Base
  belongs_to :topic
  belongs_to :post
  belongs_to :review
end

class Topic < ActiveRecord::Base
  has_many :flags
end

class Post < ActiveRecord::Base
  has_many :flags
end

class Review < ActiveRecord::Base
  has_many :flags
end

When it comes down to flagging for, let's say, a post how do I ensure that no other ID (topic_id, review_id) can exist thereafter? I thought of creating specific flag models for each of the three but that route seems too repetitive. Would a model validation be my only sensible choice? If so what would it look like? More importantly am I going about this association setup in the right way?

Answer 1

What about using polymorphic associations?

Try with:

class Flag < ActiveRecord::Base
  belongs_to :flaggable, polymorphic: true
end

class Topic < ActiveRecord::Base
  has_many :flags, as: :flaggable
end

class Post < ActiveRecord::Base
  has_many :flags, as: :flaggable
end

class Review < ActiveRecord::Base
  has_many :flags, as: :flaggable
end


class UpdateAssociationsOnFlags < ActiveRecord::Migration
  def change
    remove_index  :flags, :topic_id
    remove_column :flags, :topic_id

    remove_index  :flags, :post_id
    remove_column :flags, :post_id

    remove_index  :flags, :review_id
    remove_column :flags, :review_id


    add_column :flags, :flaggable_id, :integer
    add_column :flags, :flaggable_type, :string

    add_index :flags, :flaggable_id
  end
end

Edit, in reply to the question author's comment:

You can just add a different association:

class Flag < ActiveRecord::Base
  belongs_to :flaggable, polymorphic: true
  belongs_to :reporter, class_name: 'User', foreign_key: :reporter_id
end

class User < ActiveRecord::Base
  has_many :reported_flags, class_name: 'Flag', foreign_key: :reporter_id
end

class AddUserForeignKeyToFlags < ActiveRecord::Migration
  def change
    add_column :flags, :reporter_id, :integer
    add_index :flags, :reporter_id
  end
end