Fixed #18757, #14462, #21565 -- Reworked database-python type conversions

Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.

3 files changed, 62 insertions, 0 deletions

diff --git a/tests/from_db_value/__init__.py b/tests/from_db_value/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tests/from_db_value/__init__.py
diff --git a/tests/from_db_value/models.py b/tests/from_db_value/models.py
new file mode 100644
index 0000000000..4cc9e62168
--- /dev/null
+++ b/tests/from_db_value/models.py
@@ -0,0 +1,32 @@
+import decimal
+
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+class Cash(decimal.Decimal):
+    currency = 'USD'
+
+    def __str__(self):
+        s = super(Cash, self).__str__(self)
+        return '%s %s' % (s, self.currency)
+
+
+class CashField(models.DecimalField):
+    def __init__(self, **kwargs):
+        kwargs['max_digits'] = 20
+        kwargs['decimal_places'] = 2
+        super(CashField, self).__init__(**kwargs)
+
+    def from_db_value(self, value, connection):
+        cash = Cash(value)
+        cash.vendor = connection.vendor
+        return cash
+
+
+@python_2_unicode_compatible
+class CashModel(models.Model):
+    cash = CashField()
+
+    def __str__(self):
+        return str(self.cash)
diff --git a/tests/from_db_value/tests.py b/tests/from_db_value/tests.py
new file mode 100644
index 0000000000..69e22d4290
--- /dev/null
+++ b/tests/from_db_value/tests.py
@@ -0,0 +1,30 @@
+from django.db import connection
+from django.db.models import Max
+from django.test import TestCase
+
+from .models import CashModel, Cash
+
+
+class FromDBValueTest(TestCase):
+    def setUp(self):
+        CashModel.objects.create(cash='12.50')
+
+    def test_simple_load(self):
+        instance = CashModel.objects.get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_values(self):
+        values_list = CashModel.objects.values_list('cash', flat=True)
+        self.assertIsInstance(values_list[0], Cash)
+
+    def test_aggregation(self):
+        maximum = CashModel.objects.aggregate(m=Max('cash'))['m']
+        self.assertIsInstance(maximum, Cash)
+
+    def test_defer(self):
+        instance = CashModel.objects.defer('cash').get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_connection(self):
+        instance = CashModel.objects.get()
+        self.assertEqual(instance.cash.vendor, connection.vendor)