Mongo java driver - retrieve slice of array without any other field

问题

I have a class called user which can be simplified to:

class User {
    String[] friends;
    //Constructor etc...
}

It is stored in a mongo collections called users. I am trying to retrieve the first N elements of the friends array without anything else from the class.

Right now, I tried using the following java query:

db.getCollection("users").find(new BasicDBObject(), new BasicDBObject("friends", new BasicDBObject("$slice", N))).next();

As expected, I get a User object with the friends array slice. But it also returns all the other fields in the User class (not shown here), which I don't want.

Any idea how I can force it to send back only the friends array?

++Cheers

回答1:

Try adding "friends: true" like this:

db.getCollection("users").find(new BasicDBObject(), new BasicDBObject("friends", new BasicDBObject("$slice", N)).append("friends", true)).next();

It should have the _id and the friends fields.

来源：https://stackoverflow.com/questions/6272546/mongo-java-driver-retrieve-slice-of-array-without-any-other-field