[4.1.x] Fixed #33984 -- Reverted "Fixed #32980 -- Made models cache related managers."

This reverts 4f8c7fd9d9 and adds
two regression tests:
- test_related_manager_refresh(), and
- test_create_copy_with_m2m().

Thanks joeli for the report.
Backport of 5e0aa362d9 from main

django/contrib/contenttypes/fields.py

@@ -552,14 +552,6 @@ class ReverseGenericManyToOneDescriptor(ReverseManyToOneDescriptor):
             self.rel,
         )
 
-    @cached_property
-    def related_manager_cache_key(self):
-        # By default, GenericRel instances will be marked as hidden unless
-        # related_query_name is given (their accessor name being "+" when
-        # hidden), which would cause multiple GenericRelations declared on a
-        # single model to collide, so always use the remote field's name.
-        return self.field.get_cache_name()
-
 
 def create_generic_related_manager(superclass, rel):
     """

django/db/models/base.py

@@ -434,18 +434,11 @@ class ModelBase(type):
         return cls._meta.default_manager
 
 
-class ModelStateCacheDescriptor:
-    """
-    Upon first access, replace itself with an empty dictionary on the instance.
-    """
-
-    def __set_name__(self, owner, name):
-        self.attribute_name = name
-
+class ModelStateFieldsCacheDescriptor:
     def __get__(self, instance, cls=None):
         if instance is None:
             return self
-        res = instance.__dict__[self.attribute_name] = {}
+        res = instance.fields_cache = {}
         return res
 
 
@@ -458,20 +451,7 @@ class ModelState:
     # explicit (non-auto) PKs. This impacts validation only; it has no effect
     # on the actual save.
     adding = True
-    fields_cache = ModelStateCacheDescriptor()
-    related_managers_cache = ModelStateCacheDescriptor()
-
-    def __getstate__(self):
-        state = self.__dict__.copy()
-        if "fields_cache" in state:
-            state["fields_cache"] = self.fields_cache.copy()
-        # Manager instances stored in related_managers_cache won't necessarily
-        # be deserializable if they were dynamically created via an inner
-        # scope, e.g. create_forward_many_to_many_manager() and
-        # create_generic_related_manager().
-        if "related_managers_cache" in state:
-            state["related_managers_cache"] = {}
-        return state
+    fields_cache = ModelStateFieldsCacheDescriptor()
 
 
 class Model(metaclass=ModelBase):
@@ -633,6 +613,7 @@ class Model(metaclass=ModelBase):
         """Hook to allow choosing the attributes to pickle."""
         state = self.__dict__.copy()
         state["_state"] = copy.copy(state["_state"])
+        state["_state"].fields_cache = state["_state"].fields_cache.copy()
         # memoryview cannot be pickled, so cast it to bytes and store
         # separately.
         _memoryview_attrs = []

django/db/models/fields/related_descriptors.py

@@ -561,14 +561,6 @@ class ReverseManyToOneDescriptor:
         self.rel = rel
         self.field = rel.field
 
-    @cached_property
-    def related_manager_cache_key(self):
-        # Being able to access the manager instance precludes it from being
-        # hidden. The rel's accessor name is used to allow multiple managers
-        # to the same model to coexist. e.g. post.attached_comment_set and
-        # post.attached_link_set are separately cached.
-        return self.rel.get_cache_name()
-
     @cached_property
     def related_manager_cls(self):
         related_model = self.rel.related_model
@@ -590,11 +582,8 @@ class ReverseManyToOneDescriptor:
         """
         if instance is None:
             return self
-        key = self.related_manager_cache_key
-        instance_cache = instance._state.related_managers_cache
-        if key not in instance_cache:
-            instance_cache[key] = self.related_manager_cls(instance)
-        return instance_cache[key]
+
+        return self.related_manager_cls(instance)
 
     def _get_set_deprecation_msg_params(self):
         return (
@@ -913,17 +902,6 @@ class ManyToManyDescriptor(ReverseManyToOneDescriptor):
             reverse=self.reverse,
         )
 
-    @cached_property
-    def related_manager_cache_key(self):
-        if self.reverse:
-            # Symmetrical M2Ms won't have an accessor name, but should never
-            # end up in the reverse branch anyway, as the related_name ends up
-            # being hidden, and no public manager is created.
-            return self.rel.get_cache_name()
-        else:
-            # For forward managers, defer to the field name.
-            return self.field.get_cache_name()
-
     def _get_set_deprecation_msg_params(self):
         return (
             "%s side of a many-to-many set"