Admin change form doesn't preserve changelist filter

[sephii]

When you save a polymorphic model form in the admin, you lose the changelist filter that was defined in your URL. This is because in django/contrib/admin/templatetags/admin_urls.py, Django compares the changelist url with the current one:

def add_preserved_filters(...):
    # ...
    current_url = '%s:%s' % (match.app_name, match.url_name)
    changelist_url = 'admin:%s_%s_changelist' % (opts.app_label, opts.model_name)

    if changelist_url == current_url and '_changelist_filters' in preserved_filters:
        # Get preserved filters

Since the changelist_url is constructed based on opts.model_name, you get the "implementation" model name, not the base one, which is the one from current_url. For example if you have a base model named "BaseTree" and an implementation named "OakTree", changelist_url would be admin:tree_oaktree_changelist but current_url would be admin:tree_basetree_changelist. So the 2 URLs don't match and the changelist filters are not correctly processed.

[sephii]

FWIW I was able to get a very hackish workaround for this. Maybe this will help you understanding the problem or getting to a solution:

class PolymorphicPreserveFiltersMixin(object):
    """
    Workaround for https://github.com/chrisglass/django_polymorphic/issues/125.
    Polymorphic doesn't preserve the changelist_form filters, so it needs some
    hacking to get it back. See the :meth:`get_polymorphic_model_name` method
    for details, the rest is just copy-pasted from
    :class:`django.contrib.admin.options.BaseModelAdmin`.
    """
    def get_polymorphic_model_name(self):
        """
        Browse through model parents to see if it has a polymorphic parent. If
        so, use its name as a `model_name`, since this will be the correct name
        to use in the route.
        """
        for parent in self.model._meta.parents:
            if issubclass(parent, PolymorphicModel):
                return parent._meta.model_name

        return self.model._meta.model_name

    def response_post_save_change(self, request, obj):
        """
        Figure out where to redirect after the 'Save' button has been pressed
        when editing an existing object.
        """
        opts = self.model._meta
        opts.model_name = self.get_polymorphic_model_name()

        if self.has_change_permission(request, None):
            post_url = reverse('admin:%s_%s_changelist' %
                               (opts.app_label, opts.model_name),
                               current_app=self.admin_site.name)
            preserved_filters = self.get_preserved_filters(request)
            post_url = add_preserved_filters({
                'preserved_filters': preserved_filters,
                'opts': opts
            }, post_url)
        else:
            post_url = reverse('admin:index',
                               current_app=self.admin_site.name)
        return HttpResponseRedirect(post_url)

[d6bels]

The issue still stands. In case it helps someone, I had to adapt some from @sephii's proposal that broke some other mixins for me because it changed self.model._meta (I am using ExportActionMixin, DALFModelAdmin).
Passing only self.model._meta in opts made it work in my case.

Also make sure this comes before other mixins you may throw at your model :
class YourModelChildAdmin(PolymorphicPreserveFiltersMixin, PolymorphicChildModelAdmin, ...)

Here's my modified version with the import you may need.

from django.contrib.admin.templatetags.admin_urls import add_preserved_filters

class PolymorphicPreserveFiltersMixin:
    """
    Make filters are kept while navigating in the admin for polymorphic models
    """
    def get_polymorphic_model_name(self):
        """
        Browse through model parents to see if it has a polymorphic parent. If
        so, use its name as a `model_name`, since this will be the correct name
        to use in the route.
        """
        for parent in self.model._meta.parents:
            if issubclass(parent, PolymorphicModel):
                return parent._meta.model_name
        return self.model._meta.model_name

    def response_post_save_change(self, request, obj):
        """
        Figure out where to redirect after the 'Save' button has been pressed
        when editing an existing object.
        """
        model_name = self.get_polymorphic_model_name() 
        if self.has_change_permission(request, None):
            post_url = reverse(
                f"admin:{self.model._meta.app_label}_{model_name}_changelist",
                current_app=self.admin_site.name,
            )
            post_url = add_preserved_filters(
                {
                    'preserved_filters': self.get_preserved_filters(request),
                    'opts': self.model._meta,
                },
                post_url,
            )
        else:
            post_url = reverse("admin:index", current_app=self.admin_site.name)
        return HttpResponseRedirect(post_url)

[bckohan]

Related to #356?