Edit a many-to-many relationship (ManyToManyField) on the Django Admin change list page.

admin.py

class BookChangeList(ChangeList):
    def __init__(self, request, model, list_display, list_display_links,
                 list_filter, date_hierarchy, search_fields, list_select_related,
                 list_per_page, list_max_show_all, list_editable, model_admin):
        super().__init__(request, model, list_display, list_display_links,
                         list_filter, date_hierarchy, search_fields, list_select_related,
                         list_per_page, list_max_show_all, list_editable, model_admin)

        # these need to be defined here, and not in BookAdmin
        self.list_display = ('name', 'categories')
        self.list_display_links = ['name']
        self.list_editable = ['categories']


class BookForm(ModelForm):
    # this the bit of custom CSS we want to add
    style_text = f'columns: {int(BookCategory.objects.count()/5)}'
    # here we only need to define the field we want to be editable

    categories = ModelMultipleChoiceField(queryset=BookCategory.objects.all(),
                                          widget=StyleableCheckboxSelectMultiple(ul_attrs={"style": style_text}),
                                          required=False)


@admin.register(Book)
class BookAdmin(admin.ModelAdmin):
    ordering = ('id',)

    def get_changelist(self, request, **kwargs):
        return BookChangeList

    def get_changelist_form(self, request, **kwargs):
        return BookForm


@admin.register(BookCategory)
class BookCategoryAdmin(admin.ModelAdmin):
    list_display = ['id', 'name']

forms.py

# this subclass allows us to add a bit of our own CSS at the <ul> level
from django import forms
from django.forms.utils import flatatt
from django.utils.safestring import mark_safe


class StyleableCheckboxSelectMultiple(forms.CheckboxSelectMultiple):
    def __init__(self, attrs=None, ul_attrs=None):
        self.ul_attrs = ul_attrs
        super().__init__(attrs)

    def render(self, name, value, attrs=None, choices=None):
        html = super().render(name, value, attrs, choices)
        final_attrs = self.build_attrs(self.ul_attrs)
        return mark_safe(html.replace('<ul', f'<ul {flatatt(final_attrs)}'))

models.py

class BookCategory(models.Model):
    name = models.CharField(max_length=255)

    def __str__(self):
        return self.name


class Book(models.Model):
    name = models.CharField(max_length=255)
    categories = models.ManyToManyField('BookCategory', blank=True, null=True)

    def __str__(self):
        return self.name

Based on https://gist.github.com/jdklub/9261959 with fixes for python 3 and Django 1.11