Ordering a Many-To-Many field in Django Admin

Question

Here's my setup:

from django.contrib.auth.models import User

class Product(models.Model):
   ...
   email_users = models.ManyToManyField(User, null=True, blank=True)
   ...

[elsewhere]

class ProductAdmin(admin.ModelAdmin):
   list_display = ('name','platform')

admin.site.register(Product, ProductAdmin)

My main problem is, when I'm viewing the "Product" page in the admin section, email users are not being being ordered by their ID by default, and I'd like that to be ordered by their username.

From what I've read so far, it seems like I need to be adding:

   email_users.admin_order_field = 'xxxx'

But I'm not quite sure what the syntax is to access the username.

Answer 1

Solution above works well, but in my case I lost all attributes and customizations that my component had by default (like required, label, etc).
In some cases could be better override __init__() method in order to customize only your queryset, nothing else will change.

class ProductAdminForm(ModelForm):
   class Meta:
      model = Product
      fields = '__all__' # required in new versions

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['email_users'].queryset = (
            self.fields['email_users'].queryset.order_by('username')
        )

Answer 2

The answer was referred to in Hao Lian's comment above, essentially, this is what needed to be done:

class ProductAdminForm(ModelForm):
   email_users = forms.ModelMultipleChoiceField(queryset=User.objects.order_by('username'))

   class Meta:
      model = Product

class ProductAdmin(admin.ModelAdmin):
   list_display = ('name','platform')
   form = ProductAdminForm

admin.site.register(Product, ProductAdmin)

Mine was slightly different in the sense that I required the forms.ModelMultipleChoiceField, whereas the answer provided used forms.ModelChoiceField()