Symmetrical editing fo a M2M relation in Django admin ¶

Given:

class Team(models.Model):

    name = models.CharField(_('name'), max_length=150, unique=True)
    jobs = models.ManyToManyField(Job, verbose_name=u'Jobs', blank=True,
        related_name='teams')


@admin.register(Team)
class TeamAdmin(admin.ModelAdmin):
    filter_horizontal = ['jobs',]
    ...

Django already provides a widget for editing the relation in the Team change form.

To have a similar behaviour in the Job change form as well, proceed as follows:

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


class JobAdminForm(forms.ModelForm):
    teams = forms.ModelMultipleChoiceField(
        queryset=Team.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Teams'),
            is_stacked=False
        )
    )

    class Meta:
        model = Job
        exclude = []

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if self.instance and self.instance.pk:
            self.fields['teams'].initial = self.instance.teams.all()

    def save(self, commit=True):
        job = super().save(commit=commit)
        if commit:
            job.teams = self.cleaned_data['teams']
        else:
            old_save_m2m = self.save_m2m
            def new_save_m2m():
                old_save_m2m()
                job.teams.set(self.cleaned_data['teams'])
            self.save_m2m = new_save_m2m
        return job


@admin.register(Job)
class JobAdmin(BaseModelAdmin):
    form = JobAdminForm
    ...

Similarly, you can even add multiple M2M relation to the JobAdminForm:

class JobAdminForm(forms.ModelForm):
    users = forms.ModelMultipleChoiceField(
        queryset=User.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Users'),
            is_stacked=False
        )
    )
    teams = forms.ModelMultipleChoiceField(
        queryset=Team.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Teams'),
            is_stacked=False
        )
    )

    class Meta:
        model = Job
        exclude = []

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if self.instance and self.instance.pk:
            self.fields['users'].initial = self.instance.users.all()
            self.fields['teams'].initial = self.instance.teams.all()

    def save(self, commit=True):
        job = super().save(commit=commit)
        if commit:
            job.users = self.cleaned_data['users']
            job.teams = self.cleaned_data['teams']
        else:
            old_save_m2m = self.save_m2m
            def new_save_m2m():
                old_save_m2m()
                job.users.set(self.cleaned_data['users'])
                job.teams.set(self.cleaned_data['teams'])
            self.save_m2m = new_save_m2m
        return job

from django.urls import reverse
from django.utils.safestring import mark_safe


def list_related_items(items, render_as_list=False):
    html = ''
    if items.count() > 0:

        admin_viewname = 'admin:{app_label}_{model_name}_change'.format(
            app_label=items[0]._meta.app_label,
            model_name=items[0]._meta.model_name,
        )

        if render_as_list:
            html += '<ul>'
            for obj in items.all():
                html += '<li><a href="{url}" style="white-space: nowrap;">{name}</a></li>'.format(
                    url=reverse(admin_viewname, args=(obj.id, )),
                    name=str(obj)
                )
            html += '</ul>'
        else:
            links = []
            for obj in items.all():
                links.append('<a href="{url}" style="white-space: nowrap;">{name}</a>'.format(
                    url=reverse(admin_viewname, args=(obj.id, )),
                    name=str(obj)
                ))
            html += ', '.join(links)
    return mark_safe(HTML)

@admin.register(Job)
class JobAdmin(BaseModelAdmin):
    form = JobAdminForm
    ...
    list_display = [
       ...
       'list_teams',
    ]

    def list_teams(self, obj):
        return list_related_items(obj.teams.all())
    list_teams.short_description = _('Teams')