Question

This is next step to this question, where I need to run queries and apply filters.
Here is my Model

from google.appengine.ext import db
Class Car(db.Model):
    name=db.StringProperty()
    model=db.StringProperty()
    mileage=db.IntegerProperty()
    person = db.ReferenceProperty(Person, collection_name='person')

Class Person(db.Model):
    name=db.StringProperty()
    age=db.IntegerProperty()

Question : For a Person I want to get all the cars he own

Approach : Get all the cars and apply filter on person whose name is “Random”
I tried the following but it doesn’t work

s = Car.all()
s.filter('person.name =', 'Random') # It fails here
result = s.fetch(1)[0] # just first result for now
print result.text
print result.votes
print result.page.language

How can I make this query run?
Thank you

Answer 1

You can only use filter on indexed attributes. person.name is from other entity! In sql you would need to use join (which is impossible when data grows big), in google bigtable like in many other non-relational databases tables join is not possible. Luckily your case is very simple, you can select all cars if you know persons key:

>>> person = Person.all().filter('name =', 'Mr. Random').fetch(1)[0]
>>> cars = Car.all().filter('person =', person.key())

If you had used a more reasonable value for collection_name

Class Car(db.Model):
    ...    
    person = db.ReferenceProperty(Person, collection_name='cars_collection')

you could access all cars like this:

>>> person = Person.all().filter('name =', 'Mr. Random').fetch(1)[0]
>>> mrs_randoms_cars = person.cars_collection