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 %}

Thanks for the gist. Helped me get a better understanding on creating m2m relationships in Wagtail.

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.