Fixed #32996 -- Cached PathInfos on relations.

PathInfo values are ostensibly static over the lifetime of the object
for which they're requested, so the data can be memoized, quickly
amortising the cost over the process' duration.

1 files changed, 22 insertions, 3 deletions

diff --git a/django/db/models/fields/reverse_related.py b/django/db/models/fields/reverse_related.py
index 65950590e2..6f0c788bbd 100644
--- a/django/db/models/fields/reverse_related.py
+++ b/django/db/models/fields/reverse_related.py
@@ -71,7 +71,7 @@ class ForeignObjectRel(FieldCacheMixin):
         When filtering against this relation, return the field on the remote
         model against which the filtering should happen.
         """
-        target_fields = self.get_path_info()[-1].target_fields
+        target_fields = self.path_infos[-1].target_fields
         if len(target_fields) > 1:
             raise exceptions.FieldError("Can't use target_field for multicolumn relations.")
         return target_fields[0]
@@ -138,6 +138,18 @@ class ForeignObjectRel(FieldCacheMixin):
     def __hash__(self):
         return hash(self.identity)
 
+    def __getstate__(self):
+        state = self.__dict__.copy()
+        # Delete the path_infos cached property because it can be recalculated
+        # at first invocation after deserialization. The attribute must be
+        # removed because subclasses like ManyToOneRel may have a PathInfo
+        # which contains an intermediate M2M table that's been dynamically
+        # created and doesn't exist in the .models module.
+        # This is a reverse relation, so there is no reverse_path_infos to
+        # delete.
+        state.pop('path_infos', None)
+        return state
+
     def get_choices(
         self, include_blank=True, blank_choice=BLANK_CHOICE_DASH,
         limit_choices_to=None, ordering=(),
@@ -195,7 +207,14 @@ class ForeignObjectRel(FieldCacheMixin):
         return opts.model_name + ('_set' if self.multiple else '')
 
     def get_path_info(self, filtered_relation=None):
-        return self.field.get_reverse_path_info(filtered_relation)
+        if filtered_relation:
+            return self.field.get_reverse_path_info(filtered_relation)
+        else:
+            return self.field.reverse_path_infos
+
+    @cached_property
+    def path_infos(self):
+        return self.get_path_info()
 
     def get_cache_name(self):
         """
@@ -234,7 +253,7 @@ class ManyToOneRel(ForeignObjectRel):
         self.field_name = field_name
 
     def __getstate__(self):
-        state = self.__dict__.copy()
+        state = super().__getstate__()
         state.pop('related_model', None)
         return state