NDB，跨多个模型查询。 AppEngine上

Question

I am struggling with querying across multiple models. This is what my class structure looks like:

class User(ndb.Model):
    ...

class LogVisit(ndb.Model)
    user = ndb.KeyProperty(kind=User)
    ...

class LogOnline(ndb.Model)
    logVisit = ndb.KeyProperty(kind = LogVisit)
    ...

and I want to get a list of the user's LogOnline's

what I want to do is this:

qry = LogOnline.query(LogOnline.logvisit.get().user.get() == user)

However app engine wont allow me to use the get method within a query.

Any thoughts on the best way to go about this? Many thanks.

Answer 1

The most efficient way will be to store the user's key in the LogOnline entity. We can;t see the rest of your model to see what LogVisit adds to the whole excercise so difficult to see what LogVisit as an intermediate entity brings to the design.

Then just

 LogOnline.query().filter(LogOnline.user == user)

You will have to stop thinking in terms of SQL if you want to have scalable applications on appengine. Think in terms of pure entity relationships and don't try to normalize the data model. Intermediate entities like LogVisit tend to only be used if you need many to many relationships but are still inefficient if you have more than a few instances of them for a particular relationship.

Answer 2

You are doing it wrong.

# user variable is assumed to be a key
logonlines = [] # You can use set also
logvisits = LogVisit.query().filter(LogVisit.user == user).fetch()
for logvisit in logvisits:
    logOnlinesA = LogOnline.query().filter(LogOnline.logVisit == logvisit.key).fetch()
    logonlines.extend(logOnlinesA)

Give it a try :

logvisits = LogVisit.query().filter(LogVisit.user == user).fetch(keys_only=True)
logOnlinesA = LogOnline.query().filter(LogOnline.logVisit.in(logvisits)).fetch()