Select users to the group in group form on Django Admin.

group_users_form.py

# -*- coding: utf-8 -*-
from django.forms import ModelForm
from django import forms
from django.contrib.auth.models import Group, User

class GroupAdminForm(ModelForm):
    class Meta:
        model = Group
    
    group_users = forms.ModelMultipleChoiceField(label=u'Usuários deste Grupo', queryset=User.objects.all())

    def __init__(self, *args, **kwargs):
        super(GroupAdminForm, self).__init__(*args, **kwargs)
        users = self.instance.user_set.all()
        users_id = [user.id for user in users]
        self.fields['group_users'].initial = users_id
        
    def save(self, commit=True):
        group = super(GroupAdminForm, self).save(commit=False)
        old_save_m2m = getattr(self, 'save_m2m', lambda: None)
        def save_m2m():
            old_save_m2m()
            group.user_set.clear()
            for user in self.cleaned_data.get('group_users',[]):
                group.user_set.add(user)
        if commit:
            group.save()
            save_m2m()
        else:
            self.save_m2m = save_m2m
        return group

Do we need revert the old old_save_m2m method? Just like below

def save(self, commit=True):
        group = super(GroupAdminForm, self).save(commit=False)
        old_save_m2m = getattr(self, 'save_m2m', lambda: None)
        def save_m2m():
            old_save_m2m()
            group.user_set.clear()
            for user in self.cleaned_data.get('group_users',[]):
                group.user_set.add(user)
            self.save_m2m = old_save_m2m # Revert the save_m2m method
        if commit:
            group.save()
            save_m2m()
        else:
            self.save_m2m = save_m2m
        return group