Question

I just wanna know how made a many to many relationship between two entity´s using the low level api in the datastore, I’ve been looking but I could not find documentation that explains how to do this kind of relationship. i hope some one can help.

but if I have the entity A and entity B in a many to many relationship how can i stored in the datastore. i mean, This a good solution to store a many to many realtionship in the datastore or what is a good solution? this code is ok or is wrong?

Entity entityA = new Entity("TypeA");
entityA.setProperty("name", "nameUserA");

Entity entityB = new Entity("TypeA");
entityA.setProperty("name", "nameUserB");

ds.put(entityA);
ds.put(entityB);

Entity entityChild = new Entity("entityChild",entityAKey);
entityChild.setProperty("name","child");

 ds.put(entityChild);

Entity entityChild = new Entity("entityChild",entityBKey);
entityChild.setProperty("name","child");

 ds.put(entityChild);

Answer 1

AppEngine doesn’t provide any functionality for many-to-many relationship.

You can emulate it, by creating your own entity, with two fields:

class AtoBRelations {
  Key entityA 
  Key entityB
}

I’s common pattern to have an extra table for many-to-many relationship, even for other databases

BTW, for most cases it will be not optimal solution. Optimal solution depends on how this many-to-many relation have to be used, and there is few different solutions

for example there is second solution: you can store a list of related entities, like:

class EntityA {
   List<Key> entitiesB
}