Instantiate edit_handler and form once in ModelAdmin views

Make use of the same `edit_handler` and `form` objects during the whole
creation and edit views process. `edit_handler` is initialized and
bounded to the instance during `setup()` which fixes #5005. This also
ensures that the form is cleaned only once.

wagtail/contrib/modeladmin/tests/test_simple_modeladmin.py

@@ -265,6 +265,13 @@ class TestCreateView(TestCase, WagtailTestUtils):
                 self.assertTrue(mock_form_fields_exclude.called)
                 m.assert_called_with(Book, exclude=mock_form_fields_exclude.return_value)
 
+    def test_clean_form_once(self):
+        with mock.patch('wagtail.tests.modeladmintest.wagtail_hooks.PublisherModelAdminForm.clean') as mock_form_clean:
+            response = self.client.post('/admin/modeladmintest/publisher/create/', {'name': ''})
+            self.assertEqual(response.status_code, 200)
+
+            mock_form_clean.assert_called_once()
+
 
 class TestInspectView(TestCase, WagtailTestUtils):
     fixtures = ['modeladmintest_test.json']
@@ -393,6 +400,15 @@ class TestEditView(TestCase, WagtailTestUtils):
                 self.assertTrue(mock_form_fields_exclude.called)
                 m.assert_called_with(Book, exclude=mock_form_fields_exclude.return_value)
 
+    def test_clean_form_once(self):
+        with mock.patch('wagtail.tests.modeladmintest.wagtail_hooks.PublisherModelAdminForm.clean') as mock_form_clean:
+            publisher = Publisher.objects.create(name='Sharper Collins')
+
+            response = self.client.post('/admin/modeladmintest/publisher/edit/%d/' % publisher.pk, {'name': ''})
+            self.assertEqual(response.status_code, 200)
+
+            mock_form_clean.assert_called_once()
+
 
 class TestPageSpecificViews(TestCase, WagtailTestUtils):
     fixtures = ['modeladmintest_test.json']

wagtail/contrib/modeladmin/views.py

@@ -108,15 +108,23 @@ class WMABaseView(TemplateView):
 
 
 class ModelFormView(WMABaseView, FormView):
+    def setup(self, request, *args, **kwargs):
+        super().setup(request, *args, **kwargs)
+        self.edit_handler = self.get_edit_handler()
+
+    def get_form(self):
+        form = super().get_form()
+        self.edit_handler = self.edit_handler.bind_to(form=form)
+        return form
 
     def get_edit_handler(self):
         edit_handler = self.model_admin.get_edit_handler(
             instance=self.get_instance(), request=self.request
         )
-        return edit_handler.bind_to(model=self.model_admin.model)
+        return edit_handler.bind_to(model=self.model_admin.model, request=self.request)
 
     def get_form_class(self):
-        return self.get_edit_handler().get_form_class()
+        return self.edit_handler.get_form_class()
 
     def get_success_url(self):
         return self.index_url
@@ -136,15 +144,12 @@ class ModelFormView(WMABaseView, FormView):
             js=self.model_admin.get_form_view_extra_js()
         )
 
-    def get_context_data(self, **kwargs):
-        instance = self.get_instance()
-        edit_handler = self.get_edit_handler()
-        form = self.get_form()
-        edit_handler = edit_handler.bind_to(
-            instance=instance, request=self.request, form=form)
+    def get_context_data(self, form=None, **kwargs):
+        if form is None:
+            form = self.get_form()
         context = {
             'is_multipart': form.is_multipart(),
-            'edit_handler': edit_handler,
+            'edit_handler': self.edit_handler,
             'form': form,
         }
         context.update(kwargs)
@@ -177,7 +182,7 @@ class ModelFormView(WMABaseView, FormView):
         messages.validation_error(
             self.request, self.get_error_message(), form
         )
-        return self.render_to_response(self.get_context_data())
+        return self.render_to_response(self.get_context_data(form=form))
 
 
 class InstanceSpecificView(WMABaseView):

wagtail/tests/modeladmintest/wagtail_hooks.py

@@ -2,7 +2,7 @@ from wagtail.admin.edit_handlers import FieldPanel, ObjectList, TabbedInterface
 from wagtail.contrib.modeladmin.helpers import WagtailBackendSearchHandler
 from wagtail.contrib.modeladmin.options import (
     ModelAdmin, ModelAdminGroup, ThumbnailMixin, modeladmin_register)
-from wagtail.contrib.modeladmin.views import CreateView, IndexView
+from wagtail.contrib.modeladmin.views import CreateView, EditView, IndexView
 from wagtail.tests.testapp.models import BusinessChild, EventPage, SingleEventPage
 
 from .forms import PublisherModelAdminForm
@@ -92,9 +92,15 @@ class PublisherCreateView(CreateView):
         return PublisherModelAdminForm
 
 
+class PublisherEditView(EditView):
+    def get_form_class(self):
+        return PublisherModelAdminForm
+
+
 class PublisherModelAdmin(ModelAdmin):
     model = Publisher
     create_view_class = PublisherCreateView
+    edit_view_class = PublisherEditView
 
 
 class EventPageAdmin(ModelAdmin):