PERF: add missing index on group_users

app/models/group_user.rb

@@ -70,4 +70,5 @@ end
 # Indexes
 #
 #  index_group_users_on_group_id_and_user_id  (group_id,user_id) UNIQUE
+#  index_group_users_on_user_id_and_group_id  (user_id,group_id) UNIQUE
 #

db/migrate/20150918004206_add_user_id_group_id_index_to_group_users.rb

@@ -0,0 +1,5 @@
+class AddUserIdGroupIdIndexToGroupUsers < ActiveRecord::Migration
+  def change
+    add_index :group_users, [:user_id, :group_id], unique: true
+  end
+end