Json encode sqlalchemy models

sqla_encoder.py

class AlchemyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj.__class__, DeclarativeMeta):
            # a SQLAlchemy class
            fields = {}
            for field in [x for x in dir(obj)
                          if not x.startswith('_') and x != 'metadata']:
                data = obj.__getattribute__(field)
                if field == "discount": data = unicode(data)
                try:
                    json.dumps(data) # this will fail on non-encodable values,
                                     # like other classes
                    fields[field] = data
                except TypeError:
                    fields[field] = None
            # a json-encodable dict
            return fields

        return json.JSONEncoder.default(self, obj)