class Directory(MPTTModel, FileCollectionChildMixin):
    name = models.CharField(max_length=300, verbose_name="Name")
    alias = models.CharField(max_length=255, verbose_name="Alias",
                             null=True, blank=True)
    object_id = models.PositiveIntegerField(blank=True, null=True)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE,
                                     blank=True, null=True)
    content_object = GenericForeignKey('content_type', 'object_id')

class DirectoryAdmin:
    readonly_fields = ('name',)
    related_search_fields = (
        'name',
        'alias',
        'content_object__first_name'
    )
    related_search_mapping = {
        # can be a list []
        'content_object': settings.AUTH_USER_MODEL
    }

site.register(Directory, DirectoryAdmin)

import xplugin_generic_search.sites
xplugin_generic_search.sites.register()