Handling Conditional Group Field Display in Django Forms

The issue you're experiencing is related to how Django handles form fields that are conditionally removed. When you remove the 'groups' field from the form for users without the proper permission, Django doesn't process that field during form submission, which means the groups relationship isn't updated.

Solution: Override the save method

Instead of simply removing the field, you should override the save() method to handle the groups relationship separately based on permissions:

PYTHONCopied
class UpdateUserForm(forms.ModelForm):
    class Meta:
        model = User
        fields = ['username', 'email', 'first_name', 'last_name', 'birth_date', 'groups']
        
    birth_date = forms.DateField(
        widget=forms.DateInput(attrs={'type': 'date', 'max': date.today().strftime('%Y-%m-%d')})
    )
        
    def __init__(self, *args, **kwargs):
        self.user = kwargs.pop('user', None)
        super().__init__(*args, **kwargs)
        
        # Store whether this user has permission to change groups
        self.can_change_groups = self.user and self.user.has_perm('AppUser.change_user')
        
        if not self.can_change_groups:
            # Hide the field but don't delete it
            self.fields['groups'].widget = forms.HiddenInput()
            # Or if you really want to remove it:
            # del self.fields['groups']
    
    def save(self, commit=True):
        user = super().save(commit=False)
        
        if commit:
            user.save()
            
            # Only update groups if the user has permission
            if self.can_change_groups and 'groups' in self.cleaned_data:
                # Save the groups relationship
                self.save_m2m()  # This will save the groups relationship
            elif not self.can_change_groups:
                # Don't modify groups for users without permission
                pass
                
        return user

Alternative Approach: Use two different forms

Another approach is to use two different forms based on the user's permissions:

PYTHONCopied
class BasicUserForm(forms.ModelForm):
    class Meta:
        model = User
        fields = ['username', 'email', 'first_name', 'last_name', 'birth_date']
        
    birth_date = forms.DateField(
        widget=forms.DateInput(attrs={'type': 'date', 'max': date.today().strftime('%Y-%m-%d')})
    )

class AdminUserForm(BasicUserForm):
    class Meta(BasicUserForm.Meta):
        fields = BasicUserForm.Meta.fields + ['groups']

# In your view:
if request.user.has_perm('AppUser.change_user'):
    user_form = AdminUserForm(request.POST, instance=user_instance)
else:
    user_form = BasicUserForm(request.POST, instance=user_instance)

Understanding the Groups Relationship

The groups field is a many-to-many relationship between the User and Group models. When you call save() on a form with a many-to-many field, Django first saves the model instance and then saves the many-to-many relationships through the save_m2m() method.

When you remove the field from the form, the form's cleaned_data doesn't include the groups information, so the relationship isn't updated when you call save().

Either of the approaches above should help you maintain the groups relationship correctly while still conditionally displaying the field based on user permissions.