JPA和Google App Engine中的多对多关系

Question

I have entities A and B, and A can have set of B. The same instance of B can belong to several A. So there is classical many-to-many relation here.

In GAE there is no direct support of many-to-many relations, instead they're offering an ability to use sets of keys for related relations. So in AI will maintain set of keys of records in B.

Now the problem is - how can I query for objects of type B belonging to given object of type A and matching certain criteria? In plain SQL I would do that like:

select B.* 
from 
    B inner join A 
        on B.A_ID=A.ID 
where B.property0=criteria1 
      and B.property1=criteria2 ...
      and ...

but because I can not do JOIN then I need to do something like

select B.* 
from B 
where B.A_ID in ( ... ) 
      and B.property0=criteria1 
      and B.property1=criteria2 ...
      and ...

so the query itself can be very long because of amount of IDs.

Is there any better way?

Answer 1

If you refactor your relationship mapping you can get a better query. Instead of storing a set of keys in A, store a set of keys in B. Then you can query with

select * from B where a_id = {idOfRelevantA} and property0 = {criterion0} and property1 = {criterion1}...

This way you avoid the multiple queries that the in operator creates.

Also, beware: in will only work for a list of 30 elements or fewer.