Wagtail Inlinepanel demo erorr - wagtail

Question

I am relatively new to Django Wagtail and I was following the demo from the docs.wagtail.io website which can be found here on how to add a list of Links to a Page using an InlinePanel with Related links
I seem to have reached an error that I donot fully understand it's meaning.
The error says
AttributeError: type object 'BookPageRelatedLinks' has no attribute 'rel'
The code for the demo is as follows
from wagtail.wagtailcore.models import Orderable, Page
from modelcluster.fields import ParentalKey
from wagtail.wagtailadmin.edit_handlers import FieldPanel,InlinePanel
from django.db import models
class BookPage(Page):
# The abstract model for related links, complete with panels
class RelatedLink(models.Model):
title = models.CharField(max_length=255)
link_external = models.URLField("External link", blank=True)
panels = [
FieldPanel('title'),
FieldPanel('link_external'),
]
class Meta:
abstract = True
# The real model which combines the abstract model, an
# Orderable helper class, and what amounts to a ForeignKey link
# to the model we want to add related links to (BookPage)
class BookPageRelatedLinks(Orderable, RelatedLink):
page = ParentalKey('demo.BookPage', related_name='related_links')
content_panels = Page.content_panels + [
InlinePanel('BookPageRelatedLinks', label="Related Links"),
]
My primary objective was to learn this so I can add image links to a sidebar on a BlogPage app I am developing.

Answer 1

Your InlinePanel declaration isn't quite correct - it needs to be:
InlinePanel('related_links', label="Related Links")
Here's what's going on:
By defining a ParentalKey with related_name='related_links', you set up a one-to-many relation called related_links on BookPage. This allows you to retrieve all of the BookPageRelatedLinks objects associated with a given BookPage instance (for example, if your BookPage instance was called page, you could write page.related_links.all()).
The InlinePanel declaration then tells Wagtail to make the related_links property editable within the admin.
The reason you're getting a misleading error message is that you've defined the RelatedLink and BookPageRelatedLinks classes inside the BookPage - which is a little bit unusual, but still valid. This results in BookPageRelatedLinks being defined as a property of BookPage (i.e. BookPage.BookPageRelatedLinks). Then, when Wagtail tries to set up the InlinePanel, it retrieves that property and fails because it's not the expected type of object (it's a class definition, not a relation).
If you write your models file in the more conventional way, with the related models defined below (or above) BookPage:
class BookPage(Page):
content_panels = Page.content_panels + [
InlinePanel('BookPageRelatedLinks', label="Related Links"),
]
# The abstract model for related links, complete with panels
class RelatedLink(models.Model):
title = models.CharField(max_length=255)
link_external = models.URLField("External link", blank=True)
panels = [
FieldPanel('title'),
FieldPanel('link_external'),
]
class Meta:
abstract = True
# The real model which combines the abstract model, an
# Orderable helper class, and what amounts to a ForeignKey link
# to the model we want to add related links to (BookPage)
class BookPageRelatedLinks(Orderable, RelatedLink):
page = ParentalKey('home.BookPage', related_name='related_links')
...then you would get a more informative error: AttributeError: type object 'BookPage' has no attribute 'BookPageRelatedLinks'.