如何使用MongoDB API Java在数组中找到文档？

Question

Here's my code

ServerAddress sa = new ServerAddress("localhost", 27017);
MongoClient mongoClient = new MongoClient(sa);
MongoDatabase db = mongoClient.getDatabase("waitinglist");
MongoCollection<Document> coll = db.getCollection("users");
MongoCursor<Document> f = coll.find(eq("users.email", "marc@berger.com")).iterator();
try {
while (f.hasNext()) {
    System.out.println("Mongo Cursor: " +f.next().toJson());
}
} finally {
  f.close();
}

And here is how my collection looks:

{ 
"_id" : ObjectId("560b8b76a37991ab2d650ca9"), 
"users" : [
    {
        "firstname" : "Marc", 
        "lastname" : "Berger", 
        "email" : "marc@berger.com", 
        "phone" : "12345"
    }, 
    {
        "firstname" : "Arnold", 
        "lastname" : "Schwarzenegger", 
        "email" : "pumping@iron.com", 
        "phone" : "12345"
    }]
}

I bassically want to get the document in users where the email is equal marc@berger.com, but it returns the whole document with the array as one. I think the problem is the first parameter in the eq method but I can't find a solution on google how make that statement.

Answer 1

You use the positional $ operator in projection in order to return only the matched element of the array. This uses the .projection() method upon .find() with the MongoDB 3.x java driver:

MongoCursor<Document> f = coll.find(eq("users.email", "marc@berger.com"))
    .projection(new Document("users.$",1)).iterator();

Then all results will only return the matched array element rather than all array elements.