Wagtail Related Inline Model

example.py

from modelcluster.fields import ParentalKey
from wagtail.wagtailcore.models import Page, Orderable

# The Parent/Consuming page
class ExamplePage(Page):
    content_panels = Page.content_panels + [
        InlinePanel('related_sub_model_label', label="Ui Heading for inline model"),
    ]

# The abstract class defining the fields. Can be named whatever.
class RelatedSubFieldsClass(models.Model):
    heading = models.TextField()

    panels = [
        FieldPanel('heading'),
    ]

    class Meta:
        abstract = True

# The class that creates the relationship        
class RelatesSubFieldToPage(Orderable, RelatedSubFieldsClass):
    page = ParentalKey('ExamplePage', related_name='related_sub_model_label')