Datastore does not support joins, as this is one of the performance bottlenecks with a Relational Databases.
Actually Datastore doesn't support joins because it's not a relational database.
Joins aren't missing from Datastore because they are performance bottlenecks -- think it through for a second. If you need to fetch related records (i.e. join) you still have to do it in your own code, with multiple calls to the Datastore API. That's going to be slower than an RDBMS doing the join itself.
If the Rails people have taught us anything about databases it's that reimplementing the advantages and features of relational databases in application code is a terrible idea.