Application error: called handler method PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'user_id' cannot be null in /var/www/html/lib/Cake/Model/Datasource/DboSource.php:502 Stack trace: #0 /var/www/html/lib/Cake/Model/Datasource/DboSource.php(502): PDOStatement->execute(Array) #1 /var/www/html/lib/Cake/Model/Datasource/DboSource.php(468): DboSource->_execute('INSERT INTO `go...', Array) #2 /var/www/html/lib/Cake/Model/Datasource/DboSource.php(1132): DboSource->execute('INSERT INTO `go...') #3 /var/www/html/lib/Cake/Model/Model.php(1942): DboSource->create(Object(GroupUser), Array, Array) #4 /var/www/html/lib/Cake/Model/Model.php(1754): Model->_doSave(Array, Array) #5 /var/www/html/lib/Cake/Model/Model.php(2352): Model->save(Array, Array) #6 /var/www/html/app/Controller/GroupsController.php(530): Model->saveMany(Array) #7 [internal function]: GroupsController->requestToJoin() #8 /var/www/html/lib/Cake/Controller/Controller.php(499): ReflectionMethod->invokeArgs(Object(GroupsController), Array) #9 /var/www/html/lib/Cake/Routing/Dispatcher.php(193): Controller->invokeAction(Object(CakeRequest)) #10 /var/www/html/lib/Cake/Routing/Dispatcher.php(167): Dispatcher->_invoke(Object(GroupsController), Object(CakeRequest)) #11 /var/www/html/app/webroot/index.php(168): Dispatcher->dispatch(Object(CakeRequest), Object(CakeResponse)) #12 {main}