[1.11.x] Fixed #28188 -- Fixed crash when pickling model fields.

Regression in d2a26c1a90e837777dabdf3d67ceec4d2a70fb86.

Thanks Adam Alton for the report and test, and Adam Johnson for
suggesting the fix.

Backport of a9874d48b1b9d91988b9f299726ec4f559fb2f75 from master

3 files changed, 16 insertions, 1 deletions

diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 0af100efd9..8d40c77345 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -514,7 +514,11 @@ class Field(RegisterLookupMixin):
             # instance. The code below will create a new empty instance of
             # class self.__class__, then update its dict with self.__dict__
             # values - so, this is very close to normal pickle.
-            return _empty, (self.__class__,), self.__dict__
+            state = self.__dict__.copy()
+            # The _get_default cached_property can't be pickled due to lambda
+            # usage.
+            state.pop('_get_default', None)
+            return _empty, (self.__class__,), state
         return _load_field, (self.model._meta.app_label, self.model._meta.object_name,
                              self.name)
 
diff --git a/docs/releases/1.11.2.txt b/docs/releases/1.11.2.txt
index fd6b7083e9..558b3e40c3 100644
--- a/docs/releases/1.11.2.txt
+++ b/docs/releases/1.11.2.txt
@@ -18,3 +18,5 @@ Bugfixes
 
 * Fixed ``django.utils.http.is_safe_url()`` crash on invalid IPv6 URLs
   (:ticket:`28142`).
+
+* Fixed regression causing pickling of model fields to crash (:ticket:`28188`).
diff --git a/tests/model_fields/tests.py b/tests/model_fields/tests.py
index a9c9794a3c..00f8338128 100644
--- a/tests/model_fields/tests.py
+++ b/tests/model_fields/tests.py
@@ -1,3 +1,5 @@
+import pickle
+
 from django import forms
 from django.db import models
 from django.test import SimpleTestCase, TestCase
@@ -76,6 +78,13 @@ class BasicFieldTests(TestCase):
         self.assertIsNotNone(f1)
         self.assertNotIn(f2, (None, 1, ''))
 
+    def test_field_instance_is_picklable(self):
+        """Field instances can be pickled."""
+        field = models.Field(max_length=100, default='a string')
+        # Must be picklable with this cached property populated (#28188).
+        field._get_default
+        pickle.dumps(field)
+
 
 class ChoicesTests(SimpleTestCase):