Get array element from MongoDB using Spring Data Mongo

Question

I want to retrieve an array from MongoDB using Spring Data Mongo.

I have collection consists of users with todos:

{
    "_id" : ObjectId("584d863e439e512f4c457eea"),
    "username" : "e",
    "todos" : [
        {
            "_id" : ObjectId("584d8654439e512f4c457eeb"),
            "title" : "Officiis id sed aut molestiae id id magni.",
            "body" : "Fugit velit rem incidunt. Et ad aliquam inventore voluptatem sed omnis voluptas. Aut sint eum voluptatem.",
            "when" : ISODate("1972-09-15T04:24:08.880Z"),
            "priority" : "HIGH",
            "status" : "DEFAULT",
            "tags" : [
                "animi",
                "veniam",
                "exercitationem"
            ]
        },
        // ...
    ]
}

I have tried the following:

public List<Todo> getUserTodos(String userId)
    Query x = new Query(Criteria.where("_id").is(userId));
    x.fields().exclude("_id").include("todos");
    return mongoTemplate.find(x, Todo.class);
}

Actually, I am getting an empty ArrayList. The response in mongo cli is like:

{
    "todos": [ {/* todo1 */}, {/* todo2 */} ]
}

How to return array from MongoDB or how to unwrap it for deserializing in Java?

What error did you get ? Just fyi the _id in the query is different from what shown in the sample. — s7vr
– s7vr, Commented Jan 7, 2017 at 19:09
@SagarReddy oops, my bad. Actually I am passing userId parameter there. I will fix question. — Andrii Abramov
– Andrii Abramov, Commented Jan 7, 2017 at 19:12

Andrii Abramov · Accepted Answer · 2018-03-10 11:37:17Z

1

Well it would always return the document and not just a part of the document, when you include the field "todo" you just ensure that, that field is returned and others aren't but still it is the document which will be returned which will contain value "todos".

If you pass Todo.class it will look into a collection called todo, which isn't there, hence the empty list.

public List<Todo> getUserTodos(String userId)
    Query x = new Query(Criteria.where("_id").is(userId));
    x.fields().exclude("_id").include("todos");
    User user = mongoTemplate.findOne(x, User.class);
    return user.getTodos();
}

This same thing can be done with a map

public List<Todo> getUserTodos(String userId)
    Query x = new Query(Criteria.where("_id").is(userId));
    x.fields().exclude("_id").include("todos");
    Map user = mongoTemplate.findOne(x, Map.class);
    if(user != null && user.containsKey("todos")){
      return (List<Todo>)user.get("todos");
    }
    return null;
}

edited Mar 10, 2018 at 11:37

Andrii Abramov

10.9k12 gold badges81 silver badges107 bronze badges

answered Jan 7, 2017 at 20:18

Rahul Kumar

2,8411 gold badge24 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Andrii Abramov Over a year ago

Yes, I tried first workaround. What about performance of such query?

Rahul Kumar Over a year ago

As far as I can see it is really optimized, look at the facts, you are querying against _id which is primary key which is already indexed, you are including the field todos only, I understand it could have much data, but still it is as fast as it gets.

Collectives™ on Stack Overflow

Get array element from MongoDB using Spring Data Mongo

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related