App Engine - why are there PhoneNumber, Link, Rating etc classes?

Question

I haven't found any reason for the existence of a few of the App Engine classes. There's a PhoneNumber, a Link, a PostalAddress, a GeoPt, a Rating, etc. Why are these given special treatment? They don't seem to have any smarts - e.g. geo searching. I know Link has more space than a String property, but the rest?

See: http://code.google.com/appengine/docs/java/datastore/dataclasses.html

Answer 1

Those types are 'semantic' types. They're present in the Java API for parity with the Python API. In the Python API, they define special behaviour with regards to the .to_xml() method - for example, a PhoneNumberProperty serializes like this:

<property name="foo" type="gd:phonenumber"><gd:phoneNumber>12345-678</gd:phoneNumber></property>

Answer 2

I think they're mostly just there to cover common cases and save developers time. If a lot of apps use a phone number field, why require each developer to have to write them? A developer can still write their own if they need/want to.

Answer 3

Not sure about java, but in python the following model/code (tested on dev server) will throw BadValueError, with the message "Invalid URL: stackoverflow.com"

class foo(db.model):
    link = db.LinkProperty()

bar = foo()
bar.link = 'stackoverflow.com'

While:

bar.link = 'http://stackoverflow.com'

Works fine.

I haven't tested, but the other properties may or may not also do validation.

Answer 4

Basically using this types in your models allows to add indirect meta data to your code. This may be useful if you are working with any kind of universal renderer for your model classes or if you are performing validation of user input on your models.

For example if you are using PhoneNumber type for a field named userNumber you reflection based renderer may understand that it should automatically assign corresponding validator to text field which will represent it.

Regards, Pavel.