PolyModel class lets an application define models that
support polymorphic queries, in a more flexible way than the standard
A query produced from a
PolyModel derived class can have results that are instances
of the class or any of its subclasses.
It is defined in
google.appengine.ext.ndb.polymodel. The following example shows
the flexibility provided by the PolyModel class.
from google.appengine.ext import ndb from google.appengine.ext.ndb import polymodel class Contact(polymodel.PolyModel): phone_number = ndb.PhoneNumberProperty() address = ndb.PostalAddressProperty() class Person(Contact): first_name = ndb.StringProperty() last_name = ndb.StringProperty() mobile_number = ndb.PhoneNumberProperty() class Company(Contact): name = ndb.StringProperty() fax_number = ndb.PhoneNumberProperty() p = Person(phone_number='1-206-555-9234', address='123 First Ave., Seattle, WA, 98101', first_name='Alfred', last_name='Smith', mobile_number='1-206-555-0117') p.put() c = Company(phone_number='1-503-555-9123', address='P.O. Box 98765, Salem, OR, 97301', name='Data Solutions, LLC', fax_number='1-503-555-6622') c.put() for contact in Contact.query(): print 'Phone: %s\nAddress: %s\n\n' % (contact.phone_number, contact.address)
Contact derived from
Model instead of from
PolyModel, each class would have a different
Contact.query() would not return instances of
proper subclasses of
If you wish to retrieve only
Person.query(). You could also use
Contact.query(Contact.class_ == 'Person').
In addition to the regular Model methods, PolyModel has some interesting class methods:
_get_kind(): the name of the root class; e.g.
Person._get_kind() == 'Contact'. The root class, Contact in this example, may override this method to use a different name as the kind used in the datastore (for the entire hierarchy rooted here).
_class_name(): the name of the current class; e.g.
Person._class_name() == 'Person'. A leaf class, Person in our example, may override this method to use a different name as the class name and in the class key. A non-leaf class may also override this method, but beware: its subclasses should also override it, or else they will all use the same class name, and you will soon be very confused.
_class_key(): a list of class names giving the hierarchy. For example,
Person._class_key() == ['Contact', 'Person']. For deeper hierarchies, this will include all bases between
PolyModeland the current class, including the latter, but excluding PolyModel itself. This is the same as the value of the
class_property. Its datastore name is 'class'.
Since the class name is used in the
and this property is used to distinguish between the subclasses,
the class names (as returned by
_class_name()) should be unique
among those subclasses.