Bi-directional many-to-many relationship in django admin

README.rst

Accessing a ManyToMany relationship from both ends

Based on the article Related ManyToManyField in Django admin site, the file forms.py contains a summary of the recommended code.

The file manytomany.py contains a generic base class in order to be DRY and avoid repeating the same code in many form classes.

An example of how to use can be found in the NineCMS project commit 52c710f.

See also Configuring Many-to-many field in django admin with related_name.

forms.py

class PageTypeForm(forms.ModelForm):
    """ Override default page type form to show related blocks
    This shows both ends of m2m in admin
    First add a custom ModelMultipleChoiceField
    Remove the widget to not use the double list widget
    https://www.lasolution.be/blog/related-manytomanyfield-django-admin-site.html
    https://github.com/django/django/blob/master/django/contrib/admin/widgets.py#L24
    """
    blocks = forms.ModelMultipleChoiceField(
        ContentBlock.objects.all(),
        widget=FilteredSelectMultiple("Blocks", True),
        required=False,
    )

    def __init__(self, *args, **kwargs):
        """ Initialize form
        If this is an existing object, load related
        :param args
        :param kwargs
        :return: None
        """
        super(PageTypeForm, self).__init__(*args, **kwargs)
        if self.instance.pk:
            self.initial['blocks'] = self.instance.blocks.values_list('pk', flat=True)

    def save(self, *args, **kwargs):
        """ Handle saving of related blocks
        :param args
        :param kwargs
        :return: instance
        """
        instance = super(PageTypeForm, self).save(*args, **kwargs)
        if instance.pk:
            for block in instance.blocks.all():
                if block not in self.cleaned_data['blocks']:
                    instance.blocks.remove(block)
            for block in self.cleaned_data['blocks']:
                if block not in instance.blocks.all():
                    instance.blocks.add(block)
        return instance

    class Meta:
        """ Meta class """
        model = PageType
        fields = ['name', 'description', 'guidelines', 'url_pattern']

manytomany.py

from django import forms
from django.contrib.admin.widgets import FilteredSelectMultiple


class ModelBiMultipleChoiceField(forms.ModelMultipleChoiceField):
    """ This shows both ends of m2m in admin """
    def __init__(self, queryset, required=False, widget=None, label=None, initial=None, help_text='',
                 double_list=None, *args, **kwargs):
        """ First add a custom ModelMultipleChoiceField
        Specify a `double_list` label in order to use the double list widget
        Field name should be the same with model's m2m field
        https://www.lasolution.be/blog/related-manytomanyfield-django-admin-site.html
        https://github.com/django/django/blob/master/django/contrib/admin/widgets.py#L24
        """
        if double_list:
            widget = FilteredSelectMultiple(double_list, True)
        super(ModelBiMultipleChoiceField, self).__init__(
            queryset, required, widget, label, initial, help_text, *args, **kwargs)


class ManyToManyModelForm(forms.ModelForm):
    """ This is a generic form to use with the ModelBiMultipleChoiceField """
    def __init__(self, *args, **kwargs):
        """ Initialize form
        :param args
        :param kwargs
        :return: None
        """
        super(ManyToManyModelForm, self).__init__(*args, **kwargs)
        # If this is an existing object, load related
        if self.instance.pk:
            # browse through all form fields and pick the ModelBiMultipleChoiceField
            for field_name in self.base_fields:
                field = self.base_fields[field_name]
                if type(field).__name__ == 'ModelBiMultipleChoiceField':
                    # Get instance property dynamically
                    # field should be same name with reverse model (ie. form.blocks vs instance.blocks)
                    self.initial[field_name] = getattr(self.instance, field_name).values_list('pk', flat=True)

        # # Use the following to add an add new block icon
        # from django.db.models import ManyToManyRel
        # from django.contrib import admin
        # rel = ManyToManyRel(ContentBlock, PageType)
        # self.fields['blocks'].widget = RelatedFieldWidgetWrapper(self.fields['blocks'].widget, rel, admin.site)

    def save(self, *args, **kwargs):
        """ Handle saving of related
        :param args
        :param kwargs
        :return: instance
        """
        instance = super(ManyToManyModelForm, self).save(*args, **kwargs)
        if instance.pk:
            # browse through all form fields and pick the ModelBiMultipleChoiceField
            for field_name in self.base_fields:
                field = self.base_fields[field_name]
                if type(field).__name__ == 'ModelBiMultipleChoiceField':
                    # the m2m records, eg if model field is `blocks`, this would be `instance.blocks.all()`
                    recordset = getattr(self.instance, field_name)
                    records = recordset.all()
                    # remove records that have been removed in form
                    for record in records:
                        if record not in self.cleaned_data[field_name]:
                            recordset.remove(record)
                    # add records that have been added in form
                    for record in self.cleaned_data[field_name]:
                        if record not in records:
                            recordset.add(record)
        return instance