Support M2M validation using signals

[ldeluigi]

MyModel has one ManyToManyField that links to AnotherModel.

For validation purposes I need to check the model after m2m relationships have been set with custom logic based on the db (I can't check it before m2m fields are populated)

At the moment, I'm raising ValidationError inside a signal receiver:

@receiver(m2m_changed, sender=MyModel.relationship.through)
def validator_from_signal(sender, instance, action, reverse, model, **kwargs):
    if action == 'post_add':
        if reverse and not check_reverse(instance):
            msg = f'AnotherModel {instance.id} violates validation checks.'
            logging.error(msg)
            raise ValidationError(msg)
        elif not reverse and not check(instance):
            msg = f'MyModel {instance.id} violates validation checks.'
            logging.error(msg)
            raise ValidationError(msg)

It works, thanks to the transaction behaviour which in case of exception rollbacks everything, but if the user submits a wrong MyModel or AnotherModel (because of ManyToMany illegal relationships), in particular with the Admin Form, they get a 500 internal server error. Which isn't very user friendly...

I know this is the expected behaviour, but I'd like the Admin form to handle ValidationError(s) and/or IntegrityError(s) from signal as normal ValidationError. I can't implement this check inside the clean() method:

1. because m2m fields are not populated on the database at that stage
2. because it would only work for the admin form

What do you think?

[Mariusz Felisiak]

Thanks for the ticket, however IntegrityErrors are not masked anywhere in the admin and they shouldn't be. Moreover, signals are the last resort and could probably be avoided in you case. Please see TicketClosingReasons/UseSupportChannels for ways to get help with Django usage, because it's a support request as it stands.

[ldeluigi]

If someone stumbles upon this I managed to fix my issue with this monkeypatch of ​https://github.com/django/django/blob/eb3699ea775548a22e0407ad12bf8cbdeaf95ff5/django/contrib/admin/options.py#L1748:

from django.contrib.admin.options import *
from django.contrib.admin.exceptions import DisallowedModelAdminToField
from django.core.exceptions import (
    PermissionDenied,
    ValidationError,
)
from django.contrib.admin.utils import (
    flatten_fieldsets,
    unquote,
)
from django.forms.formsets import all_valid
from django.contrib.admin import helpers
from django.utils.translation import gettext as _

class NewModelAdmin(ModelAdmin):
    def _changeform_view(self, request, object_id, form_url, extra_context):
        to_field = request.POST.get(TO_FIELD_VAR, request.GET.get(TO_FIELD_VAR))
        if to_field and not self.to_field_allowed(request, to_field):
            raise DisallowedModelAdminToField(
                "The field %s cannot be referenced." % to_field
            )

        model = self.model
        opts = model._meta

        if request.method == "POST" and "_saveasnew" in request.POST:
            object_id = None

        add = object_id is None

        if add:
            if not self.has_add_permission(request):
                raise PermissionDenied
            obj = None

        else:
            obj = self.get_object(request, unquote(object_id), to_field)

            if request.method == "POST":
                if not self.has_change_permission(request, obj):
                    raise PermissionDenied
            else:
                if not self.has_view_or_change_permission(request, obj):
                    raise PermissionDenied

            if obj is None:
                return self._get_obj_does_not_exist_redirect(request, opts, object_id)

        fieldsets = self.get_fieldsets(request, obj)
        ModelForm = self.get_form(
            request, obj, change=not add, fields=flatten_fieldsets(fieldsets)
        )
        if request.method == "POST":
            form = ModelForm(request.POST, request.FILES, instance=obj)
            formsets, inline_instances = self._create_formsets(
                request,
                form.instance if add else obj,
                change=not add,
            )
            form_validated = form.is_valid()
            if form_validated:
                new_object = self.save_form(request, form, change=not add)
            else:
                new_object = form.instance
            if all_valid(formsets) and form_validated:
                try:
                    self.save_model(request, new_object, form, not add)
                    self.save_related(request, form, formsets, not add)
                    change_message = self.construct_change_message(
                    request, form, formsets, add
                    )
                    if add:
                        self.log_addition(request, new_object, change_message)
                        return self.response_add(request, new_object)
                    else:
                        self.log_change(request, new_object, change_message)
                        return self.response_change(request, new_object)
                except ValidationError as e:
                    form_validated = False
                    form._update_errors([e])
            else:
                form_validated = False
        else:
            if add:
                initial = self.get_changeform_initial_data(request)
                form = ModelForm(initial=initial)
                formsets, inline_instances = self._create_formsets(
                    request, form.instance, change=False
                )
            else:
                form = ModelForm(instance=obj)
                formsets, inline_instances = self._create_formsets(
                    request, obj, change=True
                )

        if not add and not self.has_change_permission(request, obj):
            readonly_fields = flatten_fieldsets(fieldsets)
        else:
            readonly_fields = self.get_readonly_fields(request, obj)
        adminForm = helpers.AdminForm(
            form,
            list(fieldsets),
            # Clear prepopulated fields on a view-only form to avoid a crash.
            self.get_prepopulated_fields(request, obj)
            if add or self.has_change_permission(request, obj)
            else {},
            readonly_fields,
            model_admin=self,
        )
        media = self.media + adminForm.media

        inline_formsets = self.get_inline_formsets(
            request, formsets, inline_instances, obj
        )
        for inline_formset in inline_formsets:
            media = media + inline_formset.media

        if add:
            title = _("Add %s")
        elif self.has_change_permission(request, obj):
            title = _("Change %s")
        else:
            title = _("View %s")
        context = {
            **self.admin_site.each_context(request),
            "title": title % opts.verbose_name,
            "subtitle": str(obj) if obj else None,
            "adminform": adminForm,
            "object_id": object_id,
            "original": obj,
            "is_popup": IS_POPUP_VAR in request.POST or IS_POPUP_VAR in request.GET,
            "to_field": to_field,
            "media": media,
            "inline_admin_formsets": inline_formsets,
            "errors": helpers.AdminErrorList(form, formsets),
            "preserved_filters": self.get_preserved_filters(request),
        }

        # Hide the "Save" and "Save and continue" buttons if "Save as New" was
        # previously chosen to prevent the interface from getting confusing.
        if (
            request.method == "POST"
            and not form_validated
            and "_saveasnew" in request.POST
        ):
            context["show_save"] = False
            context["show_save_and_continue"] = False
            # Use the change template instead of the add template.
            add = False

        context.update(extra_context or {})

        return self.render_change_form(
            request, context, add=add, change=not add, obj=obj, form_url=form_url
        )

using this modified ModelAdmin called NewModelAdmin will catch ValidationError(s) during the save process and properly handle those.

My code overrides the entire method but changes only these lines:
​https://github.com/django/django/blob/eb3699ea775548a22e0407ad12bf8cbdeaf95ff5/django/contrib/admin/options.py#L1796-L1807