Question

Is there a elegant way of using a ListProperty for storing a subclassed db.Property type?

For example, the FuzzyDateProperty from this example uses get_value_for_datastore() and make_value_from_datastore() to convert its attributes into one int that is stored in the datastore. Since that one int is a Python primitive, it seems that you should be able to create a ListProperty of FuzzyDateProperty. How?

In my particular case, i’ve defined a class and helper functions to neatly serialize / deserialize its attributes. I would like to encapsulate the class as a db.Property, rather than make the implementer handle the relationship between the class and the Model property.

Answer 1

As recommended by @mjhm and @Nick, i’ve subclassed ListProperty to accept any class. I’ve uploaded a generic version to GitHub, named ObjectListProperty. I use it as a cleaner alternative to using parallel ListProperty’s.

ObjectListProperty transparently serializes/deserializes when getting & putting the model. It has an internal serialization method that works for simple objects, but can handle more complex objects if they define their own serialization method. Here’s a trivial example:

from object_list_property import ObjectListProperty

class Animal():
    """ A simple object that we want to store with our model """
    def __init__(self, species, sex):
        self.species = species
        self.sex = sex if sex == 'male' or sex == 'female' else 'unknown'

class Zoo(db.Model):
    """ Our model contains of list of Animal's """
    mammals = ObjectListProperty(Animal, indexed=False)

class AddMammalToZoo(webapp.RequestHandler):
    def post(self):
        # Implicit in get is deserializing the ObjectListProperty items
        zoo = Zoo.all().get()

        animal = Animal(species=self.request.get('species'),
                        sex=self.request.get('sex') )

        # We can use our ObjectListProperty just like a list of object's
        zoo.mammals.append(animal)

        # Implicit in put is serializing the ObjectListProperty items
        zoo.put()