gistfile1.txt

$ cat db/migrate/* app/models/*.rb
class CreatePosts < ActiveRecord::Migration[5.1]
  def change
    create_table :posts do |t|
      t.string :title

      t.timestamps
    end
  end
end
class CreatePictures < ActiveRecord::Migration[5.1]
  def change
    create_table :pictures do |t|
      t.string :name

      t.timestamps
    end
  end
end
class CreateGalleries < ActiveRecord::Migration[5.1]
  def change
    create_table :galleries do |t|
      t.string :name

      t.timestamps
    end
  end
end
class CreateContentLinks < ActiveRecord::Migration[5.1]
  def change
    create_table :content_links do |t|
      t.references :post, foreign_key: true
      t.references :attachable, polymorphic: true

      t.timestamps
    end
  end
end
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end
class ContentLink < ApplicationRecord
  belongs_to :post
  belongs_to :attachable, polymorphic: true
  
  scope :pictures, -> { where(attachable_type: "Picture") }
  scope :galleries, -> { where(attachable_type: "Gallery") }
end
class Gallery < ApplicationRecord
  has_many :content_links, as: :attachable
end
class Picture < ApplicationRecord
  has_many :content_links, as: :attachable
end
class Post < ApplicationRecord
  has_many :content_links
end

# and then to query:
Loading development environment (Rails 5.1.4)
>> Post.first.content_links.pictures
  Post Load (0.2ms)  SELECT  "posts".* FROM "posts" ORDER BY "posts"."id" ASC LIMIT $1  [["LIMIT", 1]]
  ContentLink Load (0.3ms)  SELECT  "content_links".* FROM "content_links" WHERE "content_links"."post_id" = $1 AND "content_links"."attachable_type" = $2 LIMIT $3  [["post_id", 1], ["attachable_type", "Picture"], ["LIMIT", 11]]
=> #<ActiveRecord::AssociationRelation [#<ContentLink id: 1, post_id: 1, attachable_type: "Picture", attachable_id: 1, created_at: "2017-12-31 01:47:37", updated_at: "2017-12-31 01:47:37">]>