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.