Fixed #13844 -- Avoid converting unknown db values to float

This patch removes an unconditional float(value) conversion from db
backend default convert_values() method. This can cause problems when
aggregating over character fields for example. In addition, Oracle
and SQLite already return the bare value from their convert_values().

In the long term the converting should be done by fields, and the
fields should then call database backend specific converters when
needed. The current setup is inflexible for 3rd party fields.

Thanks to Merlijn van Deen for the original patch.

django/db/backends/__init__.py

@@ -878,19 +878,19 @@ class BaseDatabaseOperations(object):
         return self.year_lookup_bounds(value)
 
     def convert_values(self, value, field):
-        """Coerce the value returned by the database backend into a consistent type that
+        """
-        is compatible with the field type.
+        Coerce the value returned by the database backend into a consistent type
+        that is compatible with the field type.
         """
         internal_type = field.get_internal_type()
         if internal_type == 'DecimalField':
             return value
-        elif internal_type and internal_type.endswith('IntegerField') or internal_type == 'AutoField':
+        elif (internal_type and (internal_type.endswith('IntegerField')
+                                 or internal_type == 'AutoField')):
             return int(value)
         elif internal_type in ('DateField', 'DateTimeField', 'TimeField'):
             return value
-        # No field, or the field isn't known to be a decimal or integer
+        return value
-        # Default to a float
-        return float(value)
 
     def check_aggregate_support(self, aggregate_func):
         """Check that the backend supports the provided aggregate

tests/regressiontests/aggregation_regress/tests.py

@@ -866,3 +866,15 @@ class AggregationTests(TestCase):
             ['Peter Norvig'],
             lambda b: b.name
         )
+
+    def test_type_conversion(self):
+        # The database backend convert_values function should not try to covert
+        # CharFields to float. Refs #13844.
+        from django.db.models import CharField
+        from django.db import connection
+        testData = 'not_a_float_value'
+        testField = CharField()
+        self.assertEqual(
+            connection.ops.convert_values(testData, testField),
+            testData
+        )