Cast value to text on PostgreSQL if custom decoder is used (#14)

Fix #5.

Instead of registering a no-op `loads()` to avoid psycopg2's automatic decoding, we cast the value into `text`. As [the docs](http://initd.org/psycopg/docs/extras.html#json-adaptation) say, it is an efficient operation that doesn’t involve a copy.

jsonfield/fields.py

@@ -85,11 +85,15 @@ def db_type(self, connection):
         def from_db_value(self, value, expression, connection):
             if value is None:
                 return None
-            return   json.loads(value, **self.decoder_kwargs)
+            elif connection.vendor == 'postgresql' and self.decoder_kwargs.get('cls') is None:
+                return value
+            return json.loads(value, **self.decoder_kwargs)
     else:
         def from_db_value(self, value, expression, connection, context):
             if value is None:
                 return None
+            elif connection.vendor == 'postgresql' and self.decoder_kwargs.get('cls') is None:
+                return value
             return json.loads(value, **self.decoder_kwargs)
 
     def get_db_prep_value(self, value, connection=None, prepared=None):
@@ -102,6 +106,12 @@ def get_prep_value(self, value):
             return None
         return json.dumps(value, **self.encoder_kwargs)
 
+    def select_format(self, compiler, sql, params):
+        if compiler.connection.vendor == 'postgresql' and self.decoder_kwargs.get('cls') is not None:
+            # Avoid psycopg2's automatic decoding to allow custom decoder
+            return '%s::text' % sql, params
+        return super().select_format(compiler, sql, params)
+
     def value_to_string(self, obj):
         return self.value_from_object(obj)
 
@@ -185,20 +195,3 @@ def validate(self, value, model_instance):
                     v(item)
             else:
                 v(value)
-
-
-def configure_database_connection(connection, **kwargs):
-    if connection.vendor != 'postgresql':
-        return
-
-    # Ensure that psycopg does not do JSON decoding under the hood
-    # We want to be able to do our own decoding with our own options
-    import psycopg2.extras
-    if hasattr(psycopg2.extras, 'register_default_jsonb'):
-        psycopg2.extras.register_default_jsonb(
-            connection.connection,
-            globally=False,
-            loads=lambda x: x)
-
-
-connection_created.connect(configure_database_connection)

jsonfield/tests/jsonfield_test_app/models.py

@@ -1,3 +1,4 @@
+from django.contrib.postgres.fields import JSONField as PostgresJSONField
 from django.db import models
 from jsonfield.fields import JSONField
 
@@ -29,3 +30,11 @@ class CallableDefaultModel(models.Model):
 
     class Meta:
         app_label = 'jsonfield'
+
+
+class PostgresParallelModel(models.Model):
+    library_json = JSONField()
+    postgres_json = PostgresJSONField()
+
+    class Meta:
+        app_label = 'jsonfield'

jsonfield/tests/test_fields.py

@@ -1,3 +1,6 @@
+from unittest import skipUnless
+
+from django.db import connection
 from django.core import serializers
 from django.test import TestCase as DjangoTestCase
 from django.utils.encoding import force_text
@@ -162,6 +165,13 @@ def test_serializing(self):
                                            JSONFieldTestModel.objects.all())
         self.assertIn('"json": "[\\"foo\\"]"', serialized)
 
+    @skipUnless(connection.vendor == 'postgresql', 'PostgreSQL-specific test')
+    def test_work_parallel_with_postgres_json_field(self):
+        data = {'foo': 'bar'}
+        obj = PostgresParallelModel.objects.create(library_json=data, postgres_json=data)
+        obj = PostgresParallelModel.objects.get(id=obj.id)
+        self.assertEqual(obj.library_json, obj.postgres_json)
+
 
 class SavingModelsTest(DjangoTestCase):
     def test_saving_null(self):