symfony3 Dynamic Forms Populate a select field depending on the value another.

Example

This is an example to show how to change the allowed choices on a subCategory select field depending on the value of the category select field. To do that you have to make your subCategory choices dynamical for both client and server side.

1. Make the form dynamic on the client side for display / user interactions

Example of client side dynamic form (using Javascript / JQuery) :

    $('#category').change(function(){
        switch($(this).val()){
            case '1': // If category == '1'
                var choice = {
                    'choice1_1':'1_1',
                    'choice1_2':'1_2',
                    'choice1_3':'1_3',
                };
            break;
            case '2': // If category == '2'
                var choice = {
                    'choice2_1':'2_1',
                    'choice2_2':'2_2',
                    'choice2_3':'2_3',
                };
            break;
            case '3': // If category == '3'
                var choice = {
                    'choice3_1':'3_1',
                    'choice3_2':'3_2',
                    'choice3_3':'3_3',
                };        
            break;
        }
        
        var $subCategorySelect = $('#subCategory');
        
        $subCategorySelect.empty();
        $.each(choice, function(key, value) {
            $subCategorySelect.append($('<option></option>')).attr('value',value).text(key);
        });
    });

Of course you could get the choices from an AJAX call. That's not the purpose of this example.

2. Make the form dynamic on the server side for initialisation / validation

Example of server side dynamic form :

    namespace AppBundle\Form;

    use Symfony\Component\Form\AbstractType;
    use Symfony\Component\Form\FormBuilderInterface;

    use Symfony\Component\Form\Extension\Core\Type\ChoiceType;

    use Symfony\Component\Form\FormEvent;
    use Symfony\Component\Form\FormEvents;

    class MyBaseFormType extends AbstractType
    {
        /**
         * @param FormBuilderInterface $builder
         * @param array $options
         */
        public function buildForm(FormBuilderInterface $builder, array $options)
        {
            $builder
                ->add('category',ChoiceType::class,array('choices'=>array(
                        'choice1'=>'1',
                        'choice2'=>'2',
                        'choice3'=>'3',
                    )))
            ;
            
            $addSubCategoryListener = function(FormEvent $event){
                $form = $event->getForm();
                $data = $event->getData();
                
                switch($data['category']){
                    case '1': // If category == '1'
                        $choices = array(
                            'choice1_1'=>'1_1',
                            'choice1_2'=>'1_2',
                            'choice1_3'=>'1_3',
                        );
                    break;
                    case '2': // If category == '2'
                        $choices = array(
                            'choice2_1'=>'2_1',
                            'choice2_2'=>'2_2',
                            'choice2_3'=>'2_3',
                        );                        
                    break;
                    case '3': // If category == '3'
                        $choices = array(
                            'choice3_1'=>'3_1',
                            'choice3_2'=>'3_2',
                            'choice3_3'=>'3_3',
                        );                        
                    break;
                }
                
                $form->add('subCategory',ChoiceType::class,array('choices'=>$choices));
            };
            
            // This listener will adapt the form with the data passed to the form during construction :
            $builder->addEventListener(FormEvents::PRE_SET_DATA, $addSubCategoryListener);
            
            // This listener will adapt the form with the submitted data :
            $builder->addEventListener(FormEvents::PRE_SUBMIT, $addSubCategoryListener);
        }
    }