Add documentation on the base_form_class attribute

docs/advanced_topics/customisation/page_editing_interface.rst

@@ -112,3 +112,75 @@ To begin, import the ``Format`` class, ``register_image_format`` function, and o
 
 
 To unregister, call ``unregister_image_format`` with the string of the ``name`` of the ``Format`` as the only argument.
+
+.. _custom_edit_handler_forms:
+
+Customising generated forms
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. class:: wagtail.wagtailadmin.forms.WagtailAdminModelForm
+.. class:: wagtail.wagtailadmin.forms.WagtailAdminPageForm
+
+Wagtail automatically generates forms using the panels configured on the model.
+By default, this form subclasses :class:`~wagtail.wagtailadmin.forms.WagtailAdminModelForm`,
+or :class:`~wagtail.wagtailadmin.forms.WagtailAdminPageForm` for pages.
+A custom base form class can be configured by setting the :attr:`base_form_class` attribute on any model.
+Custom forms for snippets must subclass :class:`~wagtail.wagtailadmin.forms.WagtailAdminModelForm`,
+and custom forms for pages must subclass :class:`~wagtail.wagtailadmin.forms.WagtailAdminPageForm`.
+
+This can be used to add non-model fields to the form, to automatically generate field content,
+or to add custom validation logic for your models:
+
+.. code-block:: python
+
+    from django import forms
+    from wagtail.wagtailadmin.edit_handlers import FieldPanel
+    from wagtail.wagtailadmin.forms import WagtailAdminPageForm
+    from wagtail.wagtailcore.models import Page
+
+
+    class EventPageForm(WagtailAdminPageForm):
+        address = forms.CharField()
+
+        def clean(self):
+            cleaned_data = super(EventPageForm, self).clean()
+
+            # Make sure that the event starts before it ends
+            start_date = cleaned_data['start_date']
+            end_date = cleaned_data['end_date']
+            if start_date and end_date and start_date > end_date:
+                self.add_error('end_date', 'The end date must be after the start date')
+
+            return cleaned_data
+
+        def save(self, commit=True):
+            page = super(EventPageForm, self).save(commit=False)
+
+            # Update the duration field from the submitted dates
+            page.duration = (page.end_date - page.start_date).days
+
+            # Fetch the location by geocoding the address
+            page.location = geocoder.get_coordinates(self.cleaned_data['address'])
+
+            if commit:
+                page.save()
+            return page
+
+
+    class EventPage(Page):
+        start_date = models.DateField()
+        end_date = models.DateField()
+        duration = models.IntegerField()
+        location = models.CharField()
+
+        content_panels = [
+            FieldPanel('given_name'),
+            FieldPanel('family_name'),
+            FieldPanel('bio'),
+        ]
+        base_form_class = EventPageForm
+
+Wagtail will generate a new subclass of this form for the model,
+adding any fields defined in ``panels`` or ``content_panels``.
+Any fields already defined on the model will not be overridden by these automatically added fields,
+so the form field for a model field can be overridden by adding it to the custom form.

docs/reference/pages/model_reference.rst

@@ -171,6 +171,13 @@ In addition to the model fields provided, ``Page`` has many properties and metho
 
         Controls if this page can be created through the Wagtail administration. Defaults to True, and is not inherited by subclasses. This is useful when using `multi-table inheritance <https://docs.djangoproject.com/en/1.8/topics/db/models/#multi-table-inheritance>`_, to stop the base model from being created as an actual page.
 
+    .. attribute:: base_form_class
+
+        The form class used as a base for editing Pages of this type in the Wagtail page editor.
+        This attribute can be set on a model to customise the Page editor form.
+        Forms must be a subclass of :class:`~wagtail.wagtailadmin.forms.WagtailAdminPageForm`.
+        See :ref:`custom_edit_handler_forms` for more information.
+
 ``Site``
 ========