0012 - Establish rules for form processing

0012-establish-rules-for-form-processing.md

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+# 12. Establish rules for form processing
+
+Date: 2020-01-13
+
+## Status
+
+In discussion
+
+## Context
+
+While simplifying forms (https://github.com/PrestaShop/PrestaShop/issues/16482) I noticed that option forms
+don't check validation with `form->isValid()` and don't use constraints (https://symfony.com/doc/current/reference/constraints.html).
+Also I noticed that option forms were redirected after processing, which would stop constraints from working.
+
+
+Object forms were successfully using constraints.
+
+In order to make forms consistent and to make constraints usable in all forms I made changes to form processing, so instead of redirecting back to original page form was rendered.
+
+This meant after submitting the form if it redirected you to a different path from original you stayed there.
+
+
+For example index route of email options page is
+
+    admin_emails_index: /configure/advanced/emails/[GET]
+
+We can't use `POST` to save the form because in email options page there is also list, and list uses admin_emails_index `POST` for list actions.
+
+So for form saving we have
+
+    admin_emails_save_options: /configure/advanced/emails/options[POST]
+
+When you go to admin_emails_index and save the form you get redirected to admin_emails_save_options. With my changes if error occurs you stay
+at admin_emails_save_options and constraint errors are correctly displayed.
+
+This introduced a problem with PrestaShop functionalities like multishop which redirect you back to the same page after doing something.
+Because admin_emails_save_options is a `POST` route, there is no `GET` route for /configure/advanced/emails/options and after redirection you get an exception: 
+
+https://github.com/PrestaShop/PrestaShop/pull/22471#issuecomment-756738141
+
+After trying to find other solutions conclusion was:
+
+ 1) We can't have one action for all forms. (See for more info https://github.com/PrestaShop/PrestaShop/pull/17819)
+ 2) If action to save the form has different route then rendering it, we can't stay on that route after attempting to process the form
+ 3) If we redirect it means all info about form is gone
+
+Point 3 also means that constraints cannot be used. They will work and form will be validated correctly using
+`form->isValid()`, but because of redirection all data about errors is lost and errors won't be shown to user.
+Leading to situations where user won't be able to save the form but won't know why.
+
+So decision was made to not use constraints where form saving route is different from route where it's rendered.
+
+Form can be validated using form data provider (https://devdocs.prestashop.com/1.7/development/architecture/migration-guide/forms/settings-forms/#form-data-providers) and errors can be passed to user using flash messages (https://symfony.com/doc/current/controller.html#flash-messages).
+## Rules for form processing:
+
+  1) Each form must have button dedicated to it
+  2) Each form should have a separate action dedicated to it
+  3) Saving one form should not save other forms
+  4) If possible(see 5) form should use constraints to show errors to users
+  5) If `POST` action to the same page where form is rendered is not available, form should have a separate route for saving it
+and redirect back to the original route. In that case:
+   * Constraints cannot be used
+   * You must not check for form validity `form->isValid()`