Create new polymorphic model Document.

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