Added validation on PostUser, to only allow a user to have a post once

Author: davidwessman

app/models/post_user.rb

@@ -4,4 +4,5 @@ class PostUser < ActiveRecord::Base
   belongs_to :user
 
   validates :user_id, :post_id, presence: true
+  validates :user_id, uniqueness: { scope: :post_id }
 end