Present ManyToMany in Django's Admin as horizontal-filter using related_name

Question

I have the following:

class User(models.Model)
    blablabla

class Product(models.Model)
    authorized_users = models.ManyToManyField(
        User,
        related_name='shared_products',
    )

I already configured the admin of Product to show authorized_users as an horizontal filter, in order to select all the users that can edit a product.

class ProductAdmin(admin.ModelAdmin):
    filter_horizontal = (
        'authorized_users',
    )

admin.site.register(Product, ProductAdmin)

The problem is that I want to do the same in the admin of User, meaning that I want to have an horizontal filter for shared_products, in order to select the products that this user is able to edit. I have tried the following which clearly doesn't work:

class UserAdmin(admin.ModelAdmin):
    filter_horizontal = (
        'authorized_users',
    )

admin.site.register(User, UserAdmin)

Other answers I have found recomend the usage of Inlines but as I have seen they are used to edit the model instance on the other end, which is not I what I want to do.

Does someone have an idea of how to achieve this?

Answer 1

class UserAdminForm(forms.ModelForm):
  products = forms.ModelMultipleChoiceField(
    queryset=Product.objects.all(), 
    required=False,
    widget=FilteredSelectMultiple(
      verbose_name=_('Products'),
      is_stacked=False
    )
  )

  class Meta:
    model = User

  def __init__(self, *args, **kwargs):
    super(UserAdminForm, self).__init__(*args, **kwargs)

    if self.instance and self.instance.pk:
      self.fields['products'].initial = self.instance.products.all()

  def save(self, commit=True):
    user = super(UserAdminForm, self).save(commit=False)

    if commit:
      user.save()

    if user.pk:
      user.products = self.cleaned_data['products']
      self.save_m2m()

    return user

class UserAdmin(admin.ModelAdmin):
  form = UserAdminForm