After a user is saved, I need to make sure that its instance is associated with a group by default.
I have found two ways to achieve that:
Overriding the model's save() method
models.py:
from django.contrib.auth.models import AbstractUser, Group
class Person(AbstractUser):
def save(self, *args, **kwargs):
super().save(*args, **kwargs)
to_add = Group.objects.get(id=1) # get_or_create is a better option
instance.groups.add(to_add)
Capturing a post_save signal:
signals.py:
from django.conf import settings
from django.contrib.auth.models import Group
from django.db.models.signals import post_save
from django.dispatch import receiver
@receiver(
post_save,
sender=settings.AUTH_USER_MODEL,
)
def save_the_group(instance, raw, **kwargs):
if not raw:
to_add = Group.objects.get(id=1) # get_or_create is a better option
instance.groups.add(to_add)
Are these methods equal in achieving their goal?
Is there a better one in Django terms of "Good Practice"?
Update
Acquiring a better understanding of how Django works, I see that the
confusion and also the solution lie in BaseModelForm.save():
...
if commit:
# If committing, save the instance and the m2m data immediately.
self.instance.save()
self._save_m2m()
...
and in BaseModelForm._save_m2m():
...
if f.name in cleaned_data:
f.save_form_data(self.instance, cleaned_data[f.name])
...
The instance is first saved to acquire a primary key (post_save
signal emmited) and then all its many to many relations are saved based
on ModelForm.cleaned_data.
If any m2m relation has been added during the post_save signal or at
the Model.save() method, it will be removed or overridden from
BaseModelForm._save_m2m(), depending on the content of the
ModelForm.cleaned_data.
The transaction.on_commit() -which is discussed as a solution in this
asnwer later on and in a few other SO answers from which I was inspired
and got downvoted- will delay the changes in the signal until
BaseModelForm._save_m2m() has concluded its operations.
Although, in some special cases the transaction.on_commit() is very useful, in this case is an overkill, not only because it is complexing the situation in
an awkward manner (the most suitable signal is m2m_changed as explained here) but because avoiding signals altogether, is rather
good.
Therefore, I will try to give a solution that caters for both occasions:
models.py
from django.contrib.auth.models import AbstractUser, Group
class Person(AbstractUser):
def save(self, *args, **kwargs):
super().save(*args, **kwargs)
if not getattr(self, 'from_modelform', False): # This flag is created in ModelForm
<add - remove groups logic>
forms.py
from django import forms
from django.contrib.auth.forms import UserChangeForm
from django.contrib.auth.models import Group
from my_app.models import Person
class PersonChangeForm(UserChangeForm):
def clean(self):
cleaned_data = super().clean()
if self.errors:
return
group = cleaned_data['groups']
to_add = Group.objects.filter(id=1)
to_remove = Group.objects.filter(id=2)
cleaned_data['groups'] = group.union(to_add).difference(to_remove)
self.instance.from_modelform = True
return cleaned_data
class Meta:
model = Person
fields = '__all__'
This will either work with:
>>> p = Person()
>>> p.username = 'username'
>>> p.password = 'password'
>>> p.save()
or with:
from django.contrib.auth.forms import UserCreationForm
from django.contrib.auth import get_user_model
from django.forms.models import modelform_factory
user_creationform_data = {
'username': 'george',
'password1': '123!trettb',
'password2': '123!trettb',
'email': '[email protected]',
}
user_model_form = modelform_factory(
get_user_model(),
form=UserCreationForm,
)
user_creation_form = user_model_form(data=user_creationform_data)
new_user = user_creation_form.save()
Old answer
Based on either this or that SO questions along with an
article titled "How to add ManytoMany model inside a post_save
signal" the solution I turned to, is to use on_commit(func, using=None):
The function you pass in will be called immediately after a hypothetical database write made where on_commit() is called would be successfully committed.
from django.conf import settings
from django.contrib.auth.models import Group
from django.db import transaction
from django.db.models.signals import post_save
from django.dispatch import receiver
def on_transaction_commit(func):
''' Create the decorator '''
def inner(*args, **kwargs):
transaction.on_commit(lambda: func(*args, **kwargs))
return inner
@receiver(
post_save,
sender=settings.AUTH_USER_MODEL,
)
@on_transaction_commit
def group_delegation(instance, raw, **kwargs):
to_add = Group.objects.get(id=1)
instance.groups.add(to_add)
The above code does not take into account that every login causes a post_save signal.
A crucial point made in the relevant Django ticket is that the
above code will not work if a save() call is made inside an
atomic transaction together with a validation that depends on the
result of the group_delegation() function.
@transaction.atomic
def accept_group_invite(request, group_id):
validate_and_add_to_group(request.user, group_id)
# The below line would always fail in your case because the
on_commit # receiver wouldn't be called until exiting this function. if request.user.has_perm('group_permission'): do_something() ...
Django docs describe in more details the constraints under which
on_commit() successfully works.
During testing, it is crucial to use the
TransactionTestCase or the
@pytest.mark.django_db(transaction=True) decorator when testing with pytest.
This is an example of how I tested this signal.