I have a custom field in a model (in this case JSONField, but I also reproduced this with PickledObjectField). If I run a query on that model in which I defer() the custom field and then save() that model, the custom field value gets corrupted. The models.py and tests.py code below reproduces this issue.
When the save() is called, Django retrieves the deferred field through a select statement. But when it executes the update statement, it incorrectly stores that field back into the database. If you look at the value of the data column, it goes from [1,2,3] to "[1,2,3]" (yes, the change is the quotes).
#models.py
from django.db import models
from django.utils import simplejson
class JSONField(models.TextField):
__metaclass__ = models.SubfieldBase
description = "JSONField automatically serializes\deserializes values to\from JSON."
def to_python(self, value):
if value == "":
return None
if isinstance(value, basestring):
value = simplejson.loads(value)
return value
def get_db_prep_save(self, value):
if value is None: return
return simplejson.dumps(value)
def value_to_string(self, obj):
value = self._get_val_from_obj(obj)
return self.get_db_prep_value(value)
class CustomFieldModel(models.Model):
name = models.CharField(max_length=100)
data = JSONField()
#tests.py
from django.test import TestCase
from models import CustomFieldModel
class DeferRegressionTest(TestCase):
def test_defer_then_save_custom_field(self):
#store a simple list in the basic JSON field
test_model = CustomFieldModel()
test_model.name = "test"
test_model.data = [1, 2, 3]
test_model.save()
#at this point, data is of list type
self.assertEqual(type(test_model.data), type(list()))
#defer the custom JSON field
retrieved_model = CustomFieldModel.objects.defer("data").get(name="test")
#on save, django runs a select to get the deferred field and then updates it, but messes up the update statement for the deferred field
retrieved_model.save()
#now let's retrieve the whole model again
retrieved_again_model = CustomFieldModel.objects.get(name="test")
#when we get the value of the JSON field, it is no longer a list! it's been corrupted!
self.assertEqual(type(retrieved_again_model.data), type(list()))
(In [12579]) Fixed #12734. Deferred fields will now be properly converted to python when accessed. Thanks, Alex Gaynor.