@Document
public class Palace {
    @Id
    private String id;
    private Location location;
    // getter setter
}

public class Location {
    private String id;
    // getter setter
}

{
    "_id" : ObjectId("5b05387a8db58e0001d38851"),
    "location" : {
        "_id" : ObjectId("5ae712d1b2b197000132cd9b"),
    }
}

{
    "_id" : ObjectId("5b05387a8db58e0001d38851"),
    "location" : {
        "id" : "5ae712d1b2b197000132cd9b",
    }
}

public class Location {
    @Field("id")
    private String id;
    // additional attributes, getter, setter
}

getPalaceByLocationId()