Testing Django Wagtail - assert that a child of the given Page type can be created under the parent, using the supplied POST data

Question

I've defined a custom page model (a blog post) as a child of a parent model (a blog index page) and I want to test that the child can be created under its parent.

The BlogPage and BlogIndexPage models are copied from the wagtail "basic blog" example in the documentation, and works as expected.

I'm trying to follow the documentation but I get the following validation error:

AssertionError: Validation errors found when creating a cms.blogpage:
E             date:
E               This field is required.
E             intro:
E               This field is required.
E             slug:
E               This field is required.
E             title:
E               This field is required.

I suspect that I'm defining my fixture incorrectly, but I am not what the correct form is. Any help is greatly appreciated! Can someone explain why it isn't working?

fixture (apps.cms.tests.fixtures.blogPage.json):

[
  {
    "model":"wagtailcore.page",
    "pk": 1,
    "fields":{
      "date":"2022-02-28",
      "intro":"intro to the post...",
      "slug":"slug/",
      "title":"the title",
      "body":"body of the post...",
      "categories":[
        1
      ],
      "content_type": ["cms", "blogpage"],
      "depth": 2
    }
  },
  {
    "model": "cms.blogpage",
    "pk": 1,
    "fields": {}
  }
]

the test class (apps.cms.tests.test_pages.py):

class MyPageTests(WagtailPageTests):
    def setUp(self):
        self.login()
        page = BlogIndexPage(title="Home page", slug="home", path="foo", depth=1)
        page.save()

    def test_create_blog_post(self):
        cwd = Path.cwd()
        root_page = BlogIndexPage.objects.first()
        with open(f"{cwd}/lettergun/apps/cms/tests/fixtures/BlogPage.json") as json_file:
            fixture = json.load(json_file)

        # Assert that a ContentPage can be made here, with this POST data
        self.assertCanCreate(root_page, BlogPage, nested_form_data(fixture))

the models (apps.cms.models.py):

class BlogIndexPage(Page):
    template = "blog.html"
    intro = models.TextField(blank=True)

    def get_context(self, request):
        # Update context to include only published posts, ordered by reverse-chron
        context = super().get_context(request)
        blogpages = self.get_children().live().order_by("-first_published_at")
        context["blogpages"] = blogpages
        return context

    content_panels = Page.content_panels + [FieldPanel("intro", classname="full")]


class BlogPageTag(TaggedItemBase):
    content_object = ParentalKey("BlogPage", related_name="tagged_items", on_delete=models.CASCADE)


class BlogPage(Page):
    template = "blog-post.html"

    date = models.DateField("Post date")
    intro = models.CharField(max_length=250)
    body = RichTextField(blank=True)
    tags = ClusterTaggableManager(through=BlogPageTag, blank=True)
    categories = ParentalManyToManyField("cms.BlogCategory", blank=True)

    def main_image(self):
        gallery_item = self.gallery_images.first()
        if gallery_item:
            return gallery_item.image
        else:
            return None

    search_fields = Page.search_fields + [
        index.SearchField("intro"),
        index.SearchField("body"),
    ]

    content_panels = Page.content_panels + [
        MultiFieldPanel(
            [
                FieldPanel("date"),
                FieldPanel("tags"),
                FieldPanel("categories", widget=forms.CheckboxSelectMultiple),
            ],
            heading="Blog information",
        ),
        FieldPanel("intro"),
        FieldPanel("body"),
        InlinePanel("gallery_images", label="Gallery images"),
    ]