Fixing #30583 -- XML serializer doesn't handle JSONFields

[chason]

When using the XML serializer on a JSON field, it was previously trying
to serialize the raw python data object that the JSONField outputs. This
change causes the XML serializer to use json.dumps and json.loads for
serializing and deserializing JSONField data.

[chason]

I'm not 100% happy with this solution, as it creates special case logic in the XML serializer for JSONFields. However, any other solution I came up with ran into problems with how the JSON serializer (improperly?) works: instead of serializing the JSON data into a string, it includes the JSON data into the subsequent JSON data structure. This means that using the existing methods on the JSONField (i.e. value_to_string which doesn't actually output a string) and using them to serialize/deserialize the data would cause backwards compatibility problems.

I'm open to suggestions for how to improve this if the current solution is deemed sub-optimal.

[felixxm]

@chason Thanks for this patch 👍 I left some comments.

[felixxm]

You can use self.assertXMLEqual, e.g.

def test_xml_serialization(self):
    test_xml_data = (
        '<django-objects version="1.0">'
        '<object model="postgres_tests.jsonmodel">'
        '<field name="field" type="JSONField">%s'
        '</field></object></django-objects>'
    )
    for value, serialized in self.test_values:
        with self.subTest(value=value):
            ...
            self.assertXMLEqual(data, test_xml_data % serialized)
            ...

[felixxm]

I agree that this solution is quite clunky 😞, we could change to_python() and value_to_string() for JSONField, e.g.

def to_python(self, value):
    if isinstance(value, str):
      value = json.loads(value)
    return value

def value_to_string(self, obj):
    return json.dumps(self.value_from_object(obj))

but it will break JSON serialization (see TestSerialization.test_dumping and TestSerialization.test_loading).

[felixxm]

Updated in #12613.