Many to many relationships in Wagtail

m2m.py

from django.db import models

from wagtail.wagtailsnippets.models import register_snippet
from wagtail.wagtailcore.models import Page
from modelcluster.fields import ParentalKey
from wagtail.wagtailadmin.edit_handlers import InlinePanel


@register_snippet
class Category(models.Model):
    """
    There can be many categories related to many posts.
    """
    title = models.CharField(max_length=255)


class Post(Page):
    """
    There can be many posts related to many categories.
    """
    # Categories come from a different table, but it's useful to have
    # a way to retrieve them easily in the correct format.
    @property
    def categories(self):
        categories = [
            # Grabs rows from the relationship index and formats them correctly.
            # See the `PostCategoryRelationship` class below.
            n.category for n in self.post_category_relationship.all()
        ]
        return categories

    # Only inline panels work here.
    content_panels = Page.content_panels + [
        # This name comes from the "related_name" field explained below.
        InlinePanel('post_category_relationship')
    ]


class PostCategoryRelationship(models.Model):
    """

    An index of references between posts and categories.
    This is where the relationship is defined.
    
    Ex. a post with three categories will create three rows in this table.

    For instance, a post titled "Bitcoin is dead" in the categories
    "tech", "finance", and "life" will create the following rows:
    
    row = 1
    post = "Bitcoin is dead"
    category = "tech"

    row = 2
    post = "Bitcoin is dead"
    category = "finance"
    
    row = 3
    post = "Bitcoin is dead"
    category = "life"

    """

    # The post this relationship is for.
    # ParentalKey is just like ForeignKey, but needed for Wagtail's "preview"
    # function to work. More: https://github.com/torchbox/django-modelcluster
    post = ParentalKey(
        'Post',
        related_name='post_category_relationship'
    )
    # The "related name" allows us to grab rows from this table with a post.
    # It creates a new property on all posts that you can access like so:
    #
    #     my_post.post_category_relationship

    # The category related to this post
    category = models.ForeignKey(
        'Category',
        related_name="+"
    )

    panels = [
        FieldPanel('category')
    ]

m2m_unannotated.py

from django.db import models

from wagtail.wagtailsnippets.models import register_snippet
from wagtail.wagtailcore.models import Page
from modelcluster.fields import ParentalKey
from wagtail.wagtailadmin.edit_handlers import InlinePanel


@register_snippet
class Category(models.Model):
    title = models.CharField(max_length=255)


class Post(Page):
    @property
    def categories(self):
        categories = [
            n.category for n in self.post_category_relationship.all()
        ]
        return categories

    content_panels = Page.content_panels + [
        InlinePanel('post_category_relationship')
    ]


class PostCategoryRelationship(models.Model):
    post = ParentalKey(
        'Post',
        related_name='post_category_relationship'
    )
    category = models.ForeignKey(
        'Category',
        related_name="+"
    )
    panels = [
        FieldPanel('category')
    ]

template.html

{% if page.categories %}
  <ul class="categories">
    {% for category in page.categories %}
      <li>{{ category.title }}</li>
    {% endfor %}
  </ul>
{% endif %}

Thank you for sharing this, but isn't this an one to many relationship? How can we reference it from Category?

This gist is from around 5 years ago. I think Wagtail now has native support for Django's ManyToMany field.