Restore ability to use non-model fields with FieldPanel

This feature is documented in http://docs.wagtail.io/en/v1.11.1/advanced_topics/customisation/page_editing_interface.html but was broken by the EditHandler refactor #3810 - see https://github.com/wagtail/wagtail/pull/4053#issuecomment-380594092

wagtail/admin/edit_handlers.py

@@ -7,7 +7,7 @@ from django.forms.formsets import DELETION_FIELD_NAME, ORDERING_FIELD_NAME
 from django.forms.models import fields_for_model
 from django.template.loader import render_to_string
 from django.utils.encoding import force_text
-from django.utils.functional import curry
+from django.utils.functional import cached_property, curry
 from django.utils.safestring import mark_safe
 from django.utils.translation import ugettext_lazy
 from taggit.managers import TaggableManager
@@ -511,8 +511,14 @@ class FieldPanel(EditHandler):
             return [curry(comparator_class, self.db_field)]
         return []
 
-    def on_model_bound(self):
-        self.db_field = self.model._meta.get_field(self.field_name)
+    @cached_property
+    def db_field(self):
+        try:
+            model = self.model
+        except AttributeError:
+            raise ImproperlyConfigured("%r must be bound to a model before calling db_field" % self)
+
+        return model._meta.get_field(self.field_name)
 
     def on_instance_bound(self):
         self.bound_field = self.form[self.field_name]

wagtail/admin/tests/test_edit_handlers.py

@@ -376,9 +376,14 @@ class TestFieldPanel(TestCase):
         self.end_date_panel = (FieldPanel('date_to', classname='full-width')
                                .bind_to_model(EventPage))
 
-    def test_invalid_field(self):
+    def test_non_model_field(self):
+        # defining a FieldPanel for a field which isn't part of a model is OK,
+        # because it might be defined on the form instead
+        field_panel = FieldPanel('barbecue').bind_to_model(Page)
+
+        # however, accessing db_field will fail
         with self.assertRaises(FieldDoesNotExist):
-            FieldPanel('barbecue').bind_to_model(Page)
+            field_panel.db_field
 
     def test_render_as_object(self):
         form = self.EventPageForm(

wagtail/admin/tests/test_pages_views.py

@@ -682,6 +682,15 @@ class TestPageCreation(TestCase, WagtailTestUtils):
         self.assertContains(response, '<a href="#tab-promote" class="">Promote</a>')
         self.assertContains(response, '<a href="#tab-dinosaurs" class="">Dinosaurs</a>')
 
+    def test_create_page_with_non_model_field(self):
+        """
+        Test that additional fields defined on the form rather than the model are accepted and rendered
+        """
+        response = self.client.get(reverse('wagtailadmin_pages:add', args=('tests', 'formclassadditionalfieldpage', self.root_page.id)))
+        self.assertEqual(response.status_code, 200)
+        self.assertTemplateUsed(response, 'wagtailadmin/pages/create.html')
+        self.assertContains(response, "Enter SMS authentication code")
+
     def test_create_simplepage_bad_permissions(self):
         # Remove privileges from user
         self.user.is_superuser = False

wagtail/tests/testapp/forms.py

@@ -12,3 +12,18 @@ class ValidatedPageForm(WagtailAdminPageForm):
         if value != 'bar':
             raise forms.ValidationError('Field foo must be bar')
         return value
+
+
+class FormClassAdditionalFieldPageForm(WagtailAdminPageForm):
+    code = forms.CharField(
+        help_text='Enter SMS authentication code', max_length=5)
+
+    def clean(self):
+        cleaned_data = super(FormClassAdditionalFieldPageForm, self).clean()
+
+        # validate the user's code with our code check
+        code = cleaned_data['code']
+        if not code:
+            raise forms.ValidationError('Code is not valid')
+
+        return cleaned_data

wagtail/tests/testapp/migrations/0030_formclassadditionalfieldpage.py

@@ -0,0 +1,28 @@
+# Generated by Django 2.0.4 on 2018-04-12 15:10
+
+from django.db import migrations, models
+import django.db.models.deletion
+import wagtail.core.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0040_page_draft_title'),
+        ('tests', '0029_auto_20180215_1950'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='FormClassAdditionalFieldPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+                ('location', models.CharField(max_length=255)),
+                ('body', wagtail.core.fields.RichTextField(blank=True)),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+    ]

wagtail/tests/testapp/models.py

@@ -39,7 +39,7 @@ from wagtail.search import index
 from wagtail.snippets.edit_handlers import SnippetChooserPanel
 from wagtail.snippets.models import register_snippet
 
-from .forms import ValidatedPageForm
+from .forms import FormClassAdditionalFieldPageForm, ValidatedPageForm
 from .views import CustomSubmissionsListView
 
 EVENT_AUDIENCE_CHOICES = (
@@ -314,6 +314,22 @@ class HeadCountRelatedModelUsingPK(models.Model):
     panels = [FieldPanel('head_count')]
 
 
+# Override the standard WagtailAdminPageForm to add field that is not in model
+# so that we can test additional potential issues like comparing versions
+class FormClassAdditionalFieldPage(Page):
+    location = models.CharField(max_length=255)
+    body = RichTextField(blank=True)
+
+    content_panels = [
+        FieldPanel('title', classname="full title"),
+        FieldPanel('location'),
+        FieldPanel('body'),
+        FieldPanel('code'),  # not in model, see set base_form_class
+    ]
+
+    base_form_class = FormClassAdditionalFieldPageForm
+
+
 # Just to be able to test multi table inheritance
 class SingleEventPage(EventPage):
     excerpt = models.TextField(