ndb通过查询StructuredProperty返回一个StructuredProperty子属性

Question

Hi I'm confused about how to return a StructuredProperty property (mouthful):

Say I have this example from the ndb tutorial:

class Address(ndb.Model):
  type = ndb.StringProperty() # E.g., 'home', 'work'
  street = ndb.StringProperty()
  city = ndb.StringProperty()

class Contact(ndb.Model):
  name = ndb.StringProperty()
  addresses = ndb.StructuredProperty(Address, repeated=True)

guido = Contact(name='Guido',
                addresses=[Address(type='home',
                                   city='Amsterdam'),
                           Address(type='work',
                                   street='Spear St',
                                   city='SF')])

guido.put()

I want to be able to query for city Amsterdam and have it return the type "home".

so if I did the query:

Contact.query(Contact.address == Address(city='Amsterdam'))

I would want it to return Home.

Answer 1

Queries in appengine by default return the whole entity. If I understand you correctly, you just want a field of the structured property returned, rather than the whole entity.

If this is the case, the you want to read up on projection queries.

https://developers.google.com/appengine/docs/python/ndb/queries#projection