App Engine, Cross reference between two entities

i will like to have two types of entities referring to each other. but python dont know about name of second entity class in the body of f开发者_开发问答irst yet. so how shall i code.

class Business(db.Model):
  bus_contact_info_ = db.ReferenceProperty(reference_class=Business_Info)

class Business_Info (db.Model):
  my_business_ =  db.ReferenceProperty(reference_class=Business)

if you advice to use reference in only one and use the implicitly created property (which is a query object) in other. then i question the CPU quota penalty of using query vs directly using get() on key

Pleas advise how to write this code in python

Queries are a little slower, and so they do use a bit more resources. ReferenceProperty does not require reference_class. So you could always define Business like:

class Business(db.Model):
  bus_contact_info_ = db.ReferenceProperty()

There may also be better options for your datastructure too. Check out the modelling relationships article for some ideas.

Is this a one-to-one mapping? If this is a one-to-one mapping, you may be better off denormalizing your data.

Does it ever change? If not (and it is one-to-one), perhaps you could use entity groups and structure your data so that you could just directly use the keys / key names. You might be able to do this by making BusinessInfo a child of Business, then always use 'i' as the key_name. For example:

business = Business().put()
business_info = BusinessInfo(key_name='i', parent=business).put()

# Get business_info from business:
business_info = db.get(db.Key.from_path('BusinessInfo', 'i', parent=business))

# Get business from business_info:
business = db.get(business_info.parent())