Create new polymorphic model Document.

app/models/concerns/documentable.rb

@@ -0,0 +1,8 @@
+module Documentable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :documents, as: :documentable, dependent: :destroy
+  end
+
+end

app/models/document.rb

@@ -0,0 +1,9 @@
+class Document < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :documentable, polymorphic: true
+
+  validates :user_id, presence: true
+  validates :documentable_id, presence: true
+  validates :documentable_type, presence: true
+
+end