text: from copy import copy
  
  from django.contrib.contenttypes.fields import GenericRelation
  from django.core.exceptions import AppRegistryNotReady, ImproperlyConfigured
  from django.db.models import CharField, FloatField, IntegerField
  from django.db.models.signals import post_delete, post_save
  
  from mezzanine.utils.deprecation import get_related_model
  
  
  class BaseGenericRelation(GenericRelation):
      """
      Extends ``GenericRelation`` to:
  
      - Add a consistent default value for ``object_id_field`` and
        check for a ``default_related_model`` attribute which can be
        defined on subclasses as a default for the ``to`` argument.
  
      - Add one or more custom fields to the model that the relation
        field is applied to, and then call a ``related_items_changed``
        method each time related items are saved or deleted, so that a
        calculated value can be stored against the custom fields since
        aggregates aren't available for GenericRelation instances.
  
      """
  
      # Mapping of field names to model fields that will be added.
      fields = {}
  
      def __init__(self, *args, **kwargs):
          """
          Set up some defaults and check for a ``default_related_model``
          attribute for the ``to`` argument.
          """
          kwargs.setdefault("object_id_field", "object_pk")
          to = getattr(self, "default_related_model", None)
          # Avoid having both a positional arg and a keyword arg for
          # the parameter ``to``
          if to and not args:
              kwargs.setdefault("to", to)
          try:
              # Check if ``related_model`` has been modified by a subclass
              self.related_model
          except (AppRegistryNotReady, AttributeError):
              # if not, all is good
              super().__init__(*args, **kwargs)
          else:
              # otherwise, warn the user to stick to the new (as of 4.0)
              # ``default_related_model`` attribute
              raise ImproperlyConfigured(
                  "BaseGenericRelation changed the "
                  "way it handled a default ``related_model`` in mezzanine "
                  "4.0. Please override ``default_related_model`` instead "
                  "and do not tamper with django's ``related_model`` "
                  "property anymore."
              )
  
      def contribute_to_class(self, cls, name):
          """
          Add each of the names and fields in the ``fields`` attribute
          to the model the relationship field is applied to, and set up
          the related item save and delete signals for calling
          ``related_items_changed``.
          """
          for field in cls._meta.many_to_many:
              if isinstance(field, self.__class__):
                  e = "Multiple {} fields are not supported ({}.{}, {}.{})".format(
                      self.__class__.__name__,
                      cls.__name__,
                      cls.__name__,
                      name,
                      field.name,
                  )
                  raise ImproperlyConfigured(e)
          self.related_field_name = name
          super().contribute_to_class(cls, name)
          # Not applicable to abstract classes, and in fact will break.
          if not cls._meta.abstract:
              for (name_string, field) in self.fields.items():
                  if "%s" in name_string:
                      name_string = name_string % name
                  extant_fields = cls._meta._forward_fields_map
                  if name_string in extant_fields:
                      continue
                  if field.verbose_name is None:
                      field.verbose_name = self.verbose_name
                  cls.add_to_class(name_string, copy(field))
              # Add a getter function to the model we can use to retrieve
              # the field/manager by name.
              getter_name = "get_%s_name" % self.__class__.__name__.lower()
              cls.add_to_class(getter_name, lambda self: name)
  
              sender = get_related_model(self)
              post_save.connect(self._related_items_changed, sender=sender)
              post_delete.connect(self._related_items_changed, sender=sender)
  
      def _related_items_changed(self, **kwargs):
          """
          Ensure that the given related item is actually for the model
          this field applies to, and pass the instance to the real
          ``related_items_changed`` handler.
          """
          for_model = kwargs["instance"].content_type.model_class()
          if for_model and issubclass(for_model, self.model):
              instance_id = kwargs["instance"].object_pk
              try:
                  instance = for_model.objects.get(id=instance_id)
              except for_model.DoesNotExist:
                  # Instance itself was deleted - signals are irrelevant.
                  return
              if hasattr(instance, "get_content_model"):
                  instance = instance.get_content_model()
              related_manager = getattr(instance, self.related_field_name)
              self.related_items_changed(instance, related_manager)
  
      def related_items_changed(self, instance, related_manager):
          """
          Can be implemented by subclasses - called whenever the
          state of related items change, eg they're saved or deleted.
          The instance for this field and the related manager for the
          field are passed as arguments.
          """
          pass
  
      def value_from_object(self, obj):
          """
          Returns the value of this field in the given model instance.
          See: https://code.djangoproject.com/ticket/22552
          """
          return getattr(obj, self.attname).all()
  
  
  class CommentsField(BaseGenericRelation):
      """
      Stores the number of comments against the
      ``COMMENTS_FIELD_NAME_count`` field when a comment is saved or
      deleted.
      """
  
      default_related_model = "generic.ThreadedComment"
      fields = {"%s_count": IntegerField(editable=False, default=0)}
  
      def related_items_changed(self, instance, related_manager):
          """
          Stores the number of comments. A custom ``count_filter``
          queryset gets checked for, allowing managers to implement
          custom count logic.
          """
          try:
              count = related_manager.count_queryset()
          except AttributeError:
              count = related_manager.count()
          count_field_name = list(self.fields.keys())[0] % self.related_field_name
          setattr(instance, count_field_name, count)
          instance.save()
  
  
  class KeywordsField(BaseGenericRelation):
      """
      Stores the keywords as a single string into the
      ``KEYWORDS_FIELD_NAME_string`` field for convenient access when
      searching.
      """
  
      default_related_model = "generic.AssignedKeyword"
      fields = {"%s_string": CharField(editable=False, blank=True, max_length=500)}
  
      def __init__(self, *args, **kwargs):
          """
          Mark the field as editable so that it can be specified in
          admin class fieldsets and pass validation, and also so that
          it shows up in the admin form.
          """
          super().__init__(*args, **kwargs)
          self.editable = True
  
      def formfield(self, **kwargs):
          """
          Provide the custom form widget for the admin, since there
          isn't a form field mapped to ``GenericRelation`` model fields.
          """
          from mezzanine.generic.forms import KeywordsWidget
  
          kwargs["widget"] = KeywordsWidget
          return super().formfield(**kwargs)
  
      def save_form_data(self, instance, data):
          """
          The ``KeywordsWidget`` field will return data as a string of
          comma separated IDs for the ``Keyword`` model - convert these
          into actual ``AssignedKeyword`` instances. Also delete
          ``Keyword`` instances if their last related ``AssignedKeyword``
          instance is being removed.
          """
          from mezzanine.generic.models import Keyword
  
          related_manager = getattr(instance, self.name)
          # Get a list of Keyword IDs being removed.
          old_ids = [str(a.keyword_id) for a in related_manager.all()]
          new_ids = data.split(",")
          removed_ids = set(old_ids) - set(new_ids)
          # Remove current AssignedKeyword instances.
          related_manager.all().delete()
          # Convert the data into AssignedKeyword instances.
          if data:
              data = [related_manager.create(keyword_id=i) for i in new_ids]
          # Remove keywords that are no longer assigned to anything.
          Keyword.objects.delete_unused(removed_ids)
  
          getattr(instance, self.name).set(data)
  
      def contribute_to_class(self, cls, name):
          """
          Swap out any reference to ``KeywordsField`` with the
          ``KEYWORDS_FIELD_string`` field in ``search_fields``.
          """
          super().contribute_to_class(cls, name)
          string_field_name = list(self.fields.keys())[0] % self.related_field_name
          if hasattr(cls, "search_fields") and name in cls.search_fields:
              try:
                  weight = cls.search_fields[name]
              except TypeError:
                  # search_fields is a sequence.
                  index = cls.search_fields.index(name)
                  search_fields_type = type(cls.search_fields)
                  cls.search_fields = list(cls.search_fields)
                  cls.search_fields[index] = string_field_name
                  cls.search_fields = search_fields_type(cls.search_fields)
              else:
                  del cls.search_fields[name]
                  cls.search_fields[string_field_name] = weight
  
      def related_items_changed(self, instance, related_manager):
          """
          Stores the keywords as a single string for searching.
          """
          assigned = related_manager.select_related("keyword")
          keywords = " ".join(str(a.keyword) for a in assigned)
          string_field_name = list(self.fields.keys())[0] % self.related_field_name
          if getattr(instance, string_field_name) != keywords:
              setattr(instance, string_field_name, keywords)
              instance.save()
  
  
  class RatingField(BaseGenericRelation):
      """
      Stores the rating count and average against the
      ``RATING_FIELD_NAME_count`` and ``RATING_FIELD_NAME_average``
      fields when a rating is saved or deleted.
      """
  
      default_related_model = "generic.Rating"
      fields = {
          "%s_count": IntegerField(default=0, editable=False),
          "%s_sum": IntegerField(default=0, editable=False),
          "%s_average": FloatField(default=0, editable=False),
      }
  
      def related_items_changed(self, instance, related_manager):
          """
          Calculates and saves the average rating.
          """
          ratings = [r.value for r in related_manager.all()]
          count = len(ratings)
          _sum = sum(ratings)
          average = _sum / count if count > 0 else 0
          setattr(instance, "%s_count" % self.related_field_name, count)
          setattr(instance, "%s_sum" % self.related_field_name, _sum)
          setattr(instance, "%s_average" % self.related_field_name, average)
          instance.save()
  