[Django]-How do I extend the Django Group model?

If you simply subclass the Group object then by default it will create a new database table and the admin site won’t pick up any new fields.

You need to inject new fields into the existing Group:

if not hasattr(Group, 'parent'):
    field = models.ForeignKey(Group, blank=True, null=True, related_name='children')
    field.contribute_to_class(Group, 'parent')

To add methods to the Group, subclass but tag the model as proxy:

class MyGroup(Group):

    class Meta:
        proxy = True

    def myFunction(self):
        return True

I managed to use migrations with @Semprini aswer.

So i needed to create a company related field in my groups related field, so in my models i did this:

if not hasattr(Group, 'company'):
    field = models.ForeignKey(Company, on_delete=models.DO_NOTHING, null=True)
    field.contribute_to_class(Group, 'company')


class Group(Group):

    class Meta:
        proxy = True

Then i run manage.py makemigrations. This created 2 files. One with dependencies on the other, but the first one belonging to the auth app was created inside my virtual enviroment. The files look like this:

# Generated by Django 2.2.5 on 2019-10-08 16:00

from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0013_guestuser_permissions_20190919_1715'),
        ('auth', '0011_update_proxy_permissions'),
    ]

    operations = [
        migrations.AddField(
            model_name='group',
            name='company',
            field=models.ForeignKey(
                null=True, on_delete=django.db.models.deletion.DO_NOTHING, to='myapp.Company'),
        ),
    ]

The second one created in myapp migrations folder look like this:

# Generated by Django 2.2.5 on 2019-10-08 16:00

import django.contrib.auth.models
from django.db import migrations


class Migration(migrations.Migration):

    dependencies = [
        ('auth', '0012_group_company_20191008'),
        ('myapp', '0013_guestuser_permissions_20190919_1715'),
    ]

    operations = [
        migrations.CreateModel(
            name='Group',
            fields=[
            ],
            options={
                'proxy': True,
                'indexes': [],
                'constraints': [],
            },
            bases=('auth.group',),
            managers=[
                ('objects', django.contrib.auth.models.GroupManager()),
            ],
        ),
    ]

So the solution was to move the file created in my virtualenv to myapp migrations folder, before the other one generated with makemigrations, but since the migration is applied to the auth app instead of myapp i have to implement a workaround in the file. So the final file now is:

# Generated by Django 2.2.5 on 2019-10-08 16:00

from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0013_guestuser_permissions_20190919_1715'),
        ('auth', '0011_update_proxy_permissions'),
    ]

    operations = [
        migrations.AddField(
            model_name='group',
            name='company',
            field=models.ForeignKey(
                null=True, on_delete=django.db.models.deletion.DO_NOTHING, to='myapp.Company'),
        ),
    ]

    def mutate_state(self, project_state, preserve=True):
        """
        This is a workaround that allows to store ``auth``
        migration outside the directory it should be stored.
        """
        app_label = self.app_label
        self.app_label = 'auth'
        state = super(Migration, self).mutate_state(project_state, preserve)
        self.app_label = app_label
        return state

    def apply(self, project_state, schema_editor, collect_sql=False):
        """
        Same workaround as described in ``mutate_state`` method.
        """
        app_label = self.app_label
        self.app_label = 'auth'
        state = super(Migration, self).apply(project_state, schema_editor, collect_sql)
        self.app_label = app_label
        return state

The mutate an apply methods allow you to migrate to the auth app from myapp migrations.

In the second file i just change the dependencie to depend on the newly file created:

# Generated by Django 2.2.5 on 2019-10-08 16:00

import django.contrib.auth.models
from django.db import migrations


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0014_group_company_20191008'),
        ('myapp', '0013_guestuser_permissions_20190919_1715'),
    ]

    operations = [
        migrations.CreateModel(
            name='Group',
            fields=[
            ],
            options={
                'proxy': True,
                'indexes': [],
                'constraints': [],
            },
            bases=('auth.group',),
            managers=[
                ('objects', django.contrib.auth.models.GroupManager()),
            ],
        ),
    ]

For me worked solution based on:

https://docs.djangoproject.com/pl/1.11/topics/auth/customizing/#extending-user

Let me explain what I did with Groups extending default model with email alias:

First of all I created my own django application let name it

python manage.py startapp auth_custom

Code section:

In auth_custom/models.py I created object CustomGroup

from django.contrib.auth.models import Group
from django.db import models

class CustomGroup(models.Model):
        """
        Overwrites original Django Group.
        """
        def __str__(self):
            return "{}".format(self.group.name)

        group = models.OneToOneField('auth.Group', unique=True)
        email_alias = models.EmailField(max_length=70, blank=True, default="")

In auth_custom/admin.py:

from django.contrib.auth.admin import GroupAdmin as BaseGroupAdmin
from django.contrib.auth.models import Group


class GroupInline(admin.StackedInline):
    model = CustomGroup
    can_delete = False
    verbose_name_plural = 'custom groups'


class GroupAdmin(BaseGroupAdmin):
    inlines = (GroupInline, )


# Re-register GroupAdmin
admin.site.unregister(Group)
admin.site.register(Group, GroupAdmin)

After making migrations I have such result in Django Admin view.

Custom Group in Django Admin

In order to access this custom field you must type:

from django.contrib.auth.models import Group


    group = Group.objects.get(name="Admins")  # example name

    email_alias = group.customgroup.email_alias

If any mistakes please notify me, I’ll correct this answere.