Fixed #20625 -- Chainable Manager/QuerySet methods.

Additionally this patch solves the orthogonal problem that specialized
`QuerySet` like `ValuesQuerySet` didn't inherit from the current `QuerySet`
type. This wasn't an issue until now because we didn't officially support
custom `QuerySet` but it became necessary with the introduction of this new
feature.

Thanks aaugustin, akaariai, carljm, charettes, mjtamlyn, shaib and timgraham
for the reviews.

django/db/models/query.py

@@ -10,7 +10,7 @@ from django.conf import settings
 from django.core import exceptions
 from django.db import connections, router, transaction, DatabaseError, IntegrityError
 from django.db.models.constants import LOOKUP_SEP
-from django.db.models.fields import AutoField
+from django.db.models.fields import AutoField, Empty
 from django.db.models.query_utils import (Q, select_related_descend,
     deferred_class_factory, InvalidQuery)
 from django.db.models.deletion import Collector
@@ -30,10 +30,23 @@ REPR_OUTPUT_SIZE = 20
 EmptyResultSet = sql.EmptyResultSet
 
 
+def _pickle_queryset(class_bases, class_dict):
+    """
+    Used by `__reduce__` to create the initial version of the `QuerySet` class
+    onto which the output of `__getstate__` will be applied.
+
+    See `__reduce__` for more details.
+    """
+    new = Empty()
+    new.__class__ = type(class_bases[0].__name__, class_bases, class_dict)
+    return new
+
+
 class QuerySet(object):
     """
     Represents a lazy database lookup for a set of objects.
     """
+
     def __init__(self, model=None, query=None, using=None):
         self.model = model
         self._db = using
@@ -45,6 +58,13 @@ class QuerySet(object):
         self._prefetch_done = False
         self._known_related_objects = {}        # {rel_field, {pk: rel_obj}}
 
+    def as_manager(cls):
+        # Address the circular dependency between `Queryset` and `Manager`.
+        from django.db.models.manager import Manager
+        return Manager.from_queryset(cls)()
+    as_manager.queryset_only = True
+    as_manager = classmethod(as_manager)
+
     ########################
     # PYTHON MAGIC METHODS #
     ########################
@@ -70,6 +90,26 @@ class QuerySet(object):
         obj_dict = self.__dict__.copy()
         return obj_dict
 
+    def __reduce__(self):
+        """
+        Used by pickle to deal with the types that we create dynamically when
+        specialized queryset such as `ValuesQuerySet` are used in conjunction
+        with querysets that are *subclasses* of `QuerySet`.
+
+        See `_clone` implementation for more details.
+        """
+        if hasattr(self, '_specialized_queryset_class'):
+            class_bases = (
+                self._specialized_queryset_class,
+                self._base_queryset_class,
+            )
+            class_dict = {
+                '_specialized_queryset_class': self._specialized_queryset_class,
+                '_base_queryset_class': self._base_queryset_class,
+            }
+            return _pickle_queryset, (class_bases, class_dict), self.__getstate__()
+        return super(QuerySet, self).__reduce__()
+
     def __repr__(self):
         data = list(self[:REPR_OUTPUT_SIZE + 1])
         if len(data) > REPR_OUTPUT_SIZE:
@@ -528,6 +568,7 @@ class QuerySet(object):
         # Clear the result cache, in case this QuerySet gets reused.
         self._result_cache = None
     delete.alters_data = True
+    delete.queryset_only = True
 
     def _raw_delete(self, using):
         """
@@ -567,6 +608,7 @@ class QuerySet(object):
         self._result_cache = None
         return query.get_compiler(self.db).execute_sql(None)
     _update.alters_data = True
+    _update.queryset_only = False
 
     def exists(self):
         if self._result_cache is None:
@@ -886,6 +928,15 @@ class QuerySet(object):
     def _clone(self, klass=None, setup=False, **kwargs):
         if klass is None:
             klass = self.__class__
+        elif not issubclass(self.__class__, klass):
+            base_queryset_class = getattr(self, '_base_queryset_class', self.__class__)
+            class_bases = (klass, base_queryset_class)
+            class_dict = {
+                '_base_queryset_class': base_queryset_class,
+                '_specialized_queryset_class': klass,
+            }
+            klass = type(klass.__name__, class_bases, class_dict)
+
         query = self.query.clone()
         if self._sticky_filter:
             query.filter_is_sticky = True