How can I set the number of initial empty (orderable) items in an InlinePanel? - wagtail

Question

I have an Orderable model called SetListItem with a ParentalKey on a ClusterableModel called FloorWithSets. The parent FloorWithSets model defines using an InlinePanel to control adding/ordering/removing of the SetListItems. The issue I have is that the admin form automatically renders three empty SetListItems for each FloorWithSets, and I cannot find any way to control this setting.
The InlinePanel class takes parameters to, e.g. set the minimum and maximum number of items, but nothing to set the number of initial empty items rendered.
I cannot find any information about this in the Wagtail docs. I've also dug into the source for InlinePanel and EditHandler but cannot find anything I could override.
I do see from the InlinePanel template file that there is a hidden input with id ending -INITIAL_FORMS which is being rendered via self.formset.management_form. The value of this field is consistently lower than a neighbouring hidden input with id ending -TOTAL_FORMS, which makes sense. I just don't understand where the value is coming from or how to control it.
The only information I can find about this INITIAL_FORMS all seems to relate to testing, (e.g. this documentation) and I cannot see how to relate what that says to what I need.
class FloorWithSets(ClusterableModel):
page = ParentalKey(EventPage, on_delete=models.CASCADE, related_name='floor_with_sets')
FLOOR_CHOICES = [
('1', 'X'),
('2', 'Y'),
('3', 'Z'),
]
floor = models.CharField(
max_length=1,
choices=FLOOR_CHOICES,
default='1',
)
panels = [
FieldPanel('floor'),
InlinePanel('set_list', label=_("set")),
]
class SetListItem(Orderable):
floor = ParentalKey(FloorWithSets, on_delete=models.CASCADE, related_name='set_list')
artist = models.CharField(max_length=255, blank=True, verbose_name=_('artist'))
label = models.CharField(max_length=255, blank=True, verbose_name=_('label'))
start_time = models.TimeField(blank=True, null=True, verbose_name=_('start time'))
end_time = models.TimeField(blank=True, null=True, verbose_name=_('end time'))
set_list_item = FieldRowPanel([
FieldPanel('artist', classname="col6"),
FieldPanel('label', classname="col6")
])
set_list_item_details = FieldRowPanel([
FieldPanel('start_time', classname="col6"),
FieldPanel('end_time', classname="col6")
])
panels = [set_list_item, set_list_item_details]

Answer 1

I think I've found a solution. Try creating a custom Form class for your EventPage model with a custom metaclass like so:
class EventPageFormMetaclass(WagtailAdminModelFormMetaclass):
#classmethod
def child_form(cls):
return EventPageForm
class EventPageForm(WagtailAdminPageForm, metaclass=EventPageFormMetaclass):
pass
class EventPage(Page):
# Whatever you have in your model
base_form_class = EventPageForm
I believe the problem stems from the fact that the ClusterFormMetaclass is hard-coded to create instances of ClusterForm for child models. So your EventPage gets a WagtailAdminPageForm, but the FloorWithSets models gets a ClusterForm. If you stop there, it's fine, but when FloorWithSets generates it's inline panels, it does so as a ClusterForm, whose metaclass has extra_form_count set to 3, as opposed to the WagtailAdminPageForm whose metaclass has it set to 0.
So the solution above creates a new Form class, whose metaclass overrides the child_form class method to return a Form class with extra_form_count set to 0.
Whew.