About Checkbox Lists for Many-to-many Relations in Administration

If you have a many-to-many relationship with a few choices, it makes sense to list the choices as a checkbox list. To do that, use formfield_overrides as follows:

@admin.register(MyModel)
class MyModelAdmin(admin.ModelAdmin):
    formfield_overrides = {
        models.ManyToManyField: {
            "widget": forms.CheckboxSelectMultiple,
        },
    }

Just make sure not to list the many-to-many fields in raw_id_fields or autocomplete_fields.

If you want to set the CheckboxSelectMultiple widget only to one specific many-to-many relation of a model, you can overwrite the ModelForm for the admin and specify the field with that widget there.