From 5cc746206726c538c36a2830e7c068f1c8a0e7c8 Mon Sep 17 00:00:00 2001
From: Sergey Fedoseev <fedoseev.sergey@gmail.com>
Date: Fri, 4 Aug 2017 11:20:15 +0500
Subject: [PATCH] Refs #28459 -- Optimized ModelState instantiation.

---
 django/db/models/base.py          | 22 +++++++++++++++-------
 tests/model_regress/test_state.py |  8 ++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)
 create mode 100644 tests/model_regress/test_state.py

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 3cff2d2fc4..d39c0b9ea6 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -370,15 +370,23 @@ class ModelBase(type):
         return cls._meta.default_manager
 
 
+class ModelStateFieldsCacheDescriptor:
+    def __get__(self, instance, cls=None):
+        if instance is None:
+            return self
+        res = instance.fields_cache = {}
+        return res
+
+
 class ModelState:
     """Store model instance state."""
-    def __init__(self, db=None):
-        self.db = db
-        # If true, uniqueness validation checks will consider this a new, as-yet-unsaved object.
-        # Necessary for correct validation of new instances of objects with explicit (non-auto) PKs.
-        # This impacts validation only; it has no effect on the actual save.
-        self.adding = True
-        self.fields_cache = {}
+    db = None
+    # If true, uniqueness validation checks will consider this a new, unsaved
+    # object. Necessary for correct validation of new instances of objects with
+    # explicit (non-auto) PKs. This impacts validation only; it has no effect
+    # on the actual save.
+    adding = True
+    fields_cache = ModelStateFieldsCacheDescriptor()
 
 
 class Model(metaclass=ModelBase):
diff --git a/tests/model_regress/test_state.py b/tests/model_regress/test_state.py
new file mode 100644
index 0000000000..f41e4e52f4
--- /dev/null
+++ b/tests/model_regress/test_state.py
@@ -0,0 +1,8 @@
+from django.db.models.base import ModelState, ModelStateFieldsCacheDescriptor
+from django.test import SimpleTestCase
+
+
+class ModelStateTests(SimpleTestCase):
+
+    def test_fields_cache_descriptor(self):
+        self.assertIsInstance(ModelState.fields_cache, ModelStateFieldsCacheDescriptor)