Foreign entity in form into different kind of input - forms

Question

I have two entities: product and category (Symfony 2.3).
I want to create a form in which an user can choose a product by first selecting the category. A user selects the category by clicking on image, then I want to set the image's value into a hidden input, but I don't see how can I change a foreign entity choice list to a hidden input (http://symfony.com/doc/current/reference/forms/types/entity.html).
How can I accomplish this? (how to change form input to hidden)
If I set cascade validation to true, will it for example check if a category really exist. (To prevent putting products with non-existing category from malicious users) ?

Answer 1

Part 1
To do this you need to use a data transformer to do two things:
transform an entity into an identifier that is either a string or integer so a form can render it as a hidden field.
transform the string or integer identifier into the entity when the form is submitted so that the parent entity can be saved with the correct relationship
The symfony docs I linked to above (here too) actually walk though an entire example of using a data transformer with a form.
As a shameless plug (because I believe it is helpful) I have written a little tutorial on using a data transformer for a hidden field with an entity id: http://lrotherfield.com/blog/symfony2-forms-entity-as-hidden-field/
Part 2
If you are using the data transformer then you don't need to worry about malicious users. The data transformer will fail because it will not be able to reverse transform the category from the fake id. In my tutorial the transformer will throw a Symfony\Component\Form\Exception\TransformationFailedException exception.
You can also write a validator (potentially using a call back) if you wanted that checks that the submitted category is real if you want an error to show in the form. Doctrine wont allow you to persist a fake category relationship as the foreign key constraint will fail.