App Engine datastore get number of childs from multiple parents

I'm using the datastore of the Google App Engine (DataNucleus). I have two classes, one Chat and one Ratin开发者_C百科g. A chat can be rated more then ones so I created an one-many relationship by adding a list to the Chat class.

Now I want to know the number of unrated chats, so I did the following:

int numberOfChatsInStock = 0;
for(Chat chat : chats){
  if(chat.getRatings().size() == 0){
    numberOfChatsInStock++;
  }
}

It simply iterate through all chats and checks if the number of ratings is 0.

This solution works well when there are 10 chats, but when I have 500+ chats then the speed is terrible (15 sec +).

I thought maybe the size() method is faster. So I tried Query.setFilter("ratings.size() == 0") but this gives me the following error: Unsupported method while parsing expression: InvokeExpression{[PrimaryExpression{ratings}].size()}

Is there any way to increase the speed?

EDIT REACTION FROM DATANUCLEUS EXPERT:

This is Google's plugin, and they are responsible for it (and the fact that they haven't implemented that method is their issue ... I'm sure their datastore offers a way of getting a size of a collection). Their plugin only uses DataNucleus 1.1 and we currently develop 2.2 so, as you can guess, we don't have much time for the GAE/J environment.

Why is google using the 1.1 version?

You should keep a rating count on the Chat model. Then you can run a query to select all chats with 0 ratings.