'Get value from property to send to datastore.
Returns:
validated list appropriate to save in the datastore.'

def get_value_for_datastore(self, model_instance):

    value = super(ListProperty, self).get_value_for_datastore(model_instance)
    if (not value):
        return value
    value = self.validate_list_contents(value)
    if self.validator:
        self.validator(value)
    if (self.item_type == datetime.date):
        value = map(_date_to_datetime, value)
    elif (self.item_type == datetime.time):
        value = map(_time_to_datetime, value)
    return value
