How to achive Join scenario in Google Datastore ndb(Python library)

[akshar via StackOverflow]

I have two kinds:

class Professor(ndb.Model):
    name = ndb.StringProperty()
    email = ndb.StringProperty()

class Student(ndb.Model):
    professor = ndb.KeyProperty(kind=Professor)
    name = ndb.StringProperty()
    age = ndb.IntegerProperty()

I want to find all the Student entities where Professor name is "Snape". How can I do this?

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[akshar via StackOverflow]

I have two kinds:

class Professor(ndb.Model):
    name = ndb.StringProperty()
    email = ndb.StringProperty()

class Student(ndb.Model):
    professor = ndb.KeyProperty(kind=Professor)
    name = ndb.StringProperty()
    age = ndb.IntegerProperty()

I want to find all the Student entities where Professor name is "Snape". How can I do this?

Or if I take the "One To Many" example of https://cloud.google.com/appengine/articles/modeling. Then I want to find all the PhoneNumber entities where Contact.name is "scott". How to do this.

The appengine example only tells how to filter on PhoneNumber attributes. What if I want to filter on the related Kind, i.e Contact attributes.

[Askar via StackOverflow]

You can do something like this.

# professor model
class Professor(ndb.Model):
    name = ndb.StringProperty()
    email = ndb.StringProperty()

# student model
class Student(ndb.Model):
    name = ndb.StringProperty()
    age = ndb.IntegerProperty()

snape = ndb.Key(Professor, 'snape')
Professor(name="Professor Snape", email="sn...@blah.edu", key=snape).put()


st1 = Student(name="Akshar", age=18, parent=snape)
st2 = Student(name="Greg", age=19, parent=snape)
st3 = Student(name="Alex", age=18, parent=snape)

st1.put()
st2.put()
st3.put()

# so lets say give me all students that has class with Professor Snape
for student in Student.query(ancestor=snape).fetch(10):
    print student.name

# You should get something:
Akshar
Greg
Alex

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