Question

what will be the best away to transform a StringProperty to an IntegerProperty without break code and doing so dynamically without changing all entities of a kind?

Obviously assuming the data is correct.

For example:
StringProperty= u’1234′ transform to IntegerProperty=1234

Now if you modify a property in model from str to int when fetching data this property says None

thanks

Answer 1

If you’re using NDB you might also wanna try ComputedProperty:

https://docs.google.com/document/d/1dsx1hihmMXMJm8wIRu49tJR-KEng80o3wkg4Nlbqn-w/edit#heading=h.crpcpjr4rp21

well, then you could define your own property type, e.g.

class Str2IntProperty(Property):
  """A Property whose original string value will be converted to an integer."""

  def _validate(self, value):
    if not isinstance(value, (int, long, str)):
      raise datastore_errors.BadValueError('Expected integer, got %r' %
                                           (value,))
    return int(value)

  def _db_set_value(self, v, unused_p, value):
    if not isinstance(value, (str, int, long)):
      raise TypeError('Str2IntProperty %s can only be set to integer or string values; '
                      'received %r' % (self._name, value))
    v.set_int64value(int(value))

  def _db_get_value(self, v, unused_p):
    if not v.has_int64value():
      try:
        return int(v)
      except ValueError:
        return 0
    return int(v.int64value())

And then in your model

class MyModel(ndb.Model):
  my_prop = Str2IntProperty()