diff --git a/app/models/group_user.rb b/app/models/group_user.rb
index 5febb87f3..a39a47056 100644
--- a/app/models/group_user.rb
+++ b/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
 #
diff --git a/db/migrate/20150918004206_add_user_id_group_id_index_to_group_users.rb b/db/migrate/20150918004206_add_user_id_group_id_index_to_group_users.rb
new file mode 100644
index 000000000..bf2058314
--- /dev/null
+++ b/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