[3.1.x] Fixed #31956 -- Fixed crash of ordering by JSONField with a custom decoder on PostgreSQL.

Thanks Marc Debureaux for the report.
Thanks Simon Charette, Nick Pope, and Adam Johnson for reviews.

Backport of 0be51d2226 from master

django/contrib/postgres/aggregates/general.py

@@ -45,7 +45,7 @@ class JSONBAgg(Aggregate):
 
     def convert_value(self, value, expression, connection):
         if not value:
-            return []
+            return '[]'
         return value
 
 

django/db/backends/base/operations.py

@@ -159,13 +159,6 @@ class BaseDatabaseOperations:
         """
         return self.date_extract_sql(lookup_type, field_name)
 
-    def json_cast_text_sql(self, field_name):
-        """Return the SQL to cast a JSON value to text value."""
-        raise NotImplementedError(
-            'subclasses of BaseDatabaseOperations may require a '
-            'json_cast_text_sql() method'
-        )
-
     def deferrable_sql(self):
         """
         Return the SQL to make a constraint "initially deferred" during a

django/db/backends/postgresql/base.py

@@ -200,7 +200,10 @@ class DatabaseWrapper(BaseDatabaseWrapper):
             # Set the isolation level to the value from OPTIONS.
             if self.isolation_level != connection.isolation_level:
                 connection.set_session(isolation_level=self.isolation_level)
-
+        # Register dummy loads() to avoid a round trip from psycopg2's decode
+        # to json.dumps() to json.loads(), when using a custom decoder in
+        # JSONField.
+        psycopg2.extras.register_default_jsonb(conn_or_curs=connection, loads=lambda x: x)
         return connection
 
     def ensure_timezone(self):

django/db/backends/postgresql/operations.py

@@ -74,9 +74,6 @@ class DatabaseOperations(BaseDatabaseOperations):
     def time_trunc_sql(self, lookup_type, field_name):
         return "DATE_TRUNC('%s', %s)::time" % (lookup_type, field_name)
 
-    def json_cast_text_sql(self, field_name):
-        return '(%s)::text' % field_name
-
     def deferrable_sql(self):
         return " DEFERRABLE INITIALLY DEFERRED"
 

django/db/models/fields/json.py

@@ -70,8 +70,6 @@ class JSONField(CheckFieldDefaultMixin, Field):
     def from_db_value(self, value, expression, connection):
         if value is None:
             return value
-        if connection.features.has_native_json_field and self.decoder is None:
-            return value
         try:
             return json.loads(value, cls=self.decoder)
         except json.JSONDecodeError:
@@ -91,14 +89,6 @@ class JSONField(CheckFieldDefaultMixin, Field):
             return transform
         return KeyTransformFactory(name)
 
-    def select_format(self, compiler, sql, params):
-        if (
-            compiler.connection.features.has_native_json_field and
-            self.decoder is not None
-        ):
-            return compiler.connection.ops.json_cast_text_sql(sql), params
-        return super().select_format(compiler, sql, params)
-
     def validate(self, value, model_instance):
         super().validate(value, model_instance)
         try:

docs/releases/3.1.1.txt

@@ -51,3 +51,7 @@ Bugfixes
 
 * Fixed detecting an async ``get_response`` callable in various builtin
   middlewares (:ticket:`31928`).
+
+* Fixed a ``QuerySet.order_by()`` crash on PostgreSQL when ordering and
+  grouping by :class:`~django.db.models.JSONField` with a custom
+  :attr:`~django.db.models.JSONField.decoder` (:ticket:`31956`).

tests/backends/base/test_operations.py

@@ -121,11 +121,6 @@ class SimpleDatabaseOperationTests(SimpleTestCase):
         with self.assertRaisesMessage(NotImplementedError, self.may_require_msg % 'datetime_extract_sql'):
             self.ops.datetime_extract_sql(None, None, None)
 
-    def test_json_cast_text_sql(self):
-        msg = self.may_require_msg % 'json_cast_text_sql'
-        with self.assertRaisesMessage(NotImplementedError, msg):
-            self.ops.json_cast_text_sql(None)
-
 
 class DatabaseOperationTests(TestCase):
     def setUp(self):

tests/model_fields/test_jsonfield.py

@@ -336,6 +336,18 @@ class TestQuerying(TestCase):
         ).values('value__d__0').annotate(count=Count('value__d__0')).order_by('count')
         self.assertQuerysetEqual(qs, [1, 11], operator.itemgetter('count'))
 
+    def test_order_grouping_custom_decoder(self):
+        NullableJSONModel.objects.create(value_custom={'a': 'b'})
+        qs = NullableJSONModel.objects.filter(value_custom__isnull=False)
+        self.assertSequenceEqual(
+            qs.values(
+                'value_custom__a',
+            ).annotate(
+                count=Count('id'),
+            ).order_by('value_custom__a'),
+            [{'value_custom__a': 'b', 'count': 1}],
+        )
+
     def test_key_transform_raw_expression(self):
         expr = RawSQL(self.raw_sql, ['{"x": "bar"}'])
         self.assertSequenceEqual(