MongoDB: Getting document and only one object of an array

Question

I got multiple documents like this:

{
    "_id" : "sgG6G9XTvvjj7uxwQ",
    "title" : "A title",
    "notes" : [
        {
            "id" : "ePce6fBAHx9KeKjuM",
            "timestamp" : 1453731403807,
            "message" : "some text",
            "name" : "Tom"
        },
        {
            "id" : "BAHx9KeKjuMePce6f",
            "timestamp" : 1453731403808,
            "message" : "some text",
            "name" : "Bob"
        },
        {
            "id" : "ePce6fBAHx9KeKjuM",
            "timestamp" : 1453731403809,
            "message" : "some text",
            "name" : "Tom"
        }
    ]
}

I get this document by using this find-query:

Collection.find({}, { sort: { title: 1 }});

But I don't need the complete notes field. I'm only interested in the id of the last note-object. That means I have to order all notes object by timestamp and just take the first (=newest) one.

I'm not quite sure, if I can do that by the find-query or if I have to do that after getting the complete data.

So the best result for me would be:

{
    "_id" : "sgG6G9XTvvjj7uxwQ",
    "title" : "A title",
    "notes" : [
        {
            "id" : "ePce6fBAHx9KeKjuM"
        }
    ]
}

Owain Williams · Accepted Answer · 2016-01-25 17:00:22Z

3

You can use the $slice projection to limit the number of array elements returned. For example:

db.collection.find({}, {title: 1, "notes.id": 1, notes: {$slice: 1}}).sort({title: 1});

Will return:

{
        "_id" : "sgG6G9XTvvjj7uxwQ",
        "title" : "A title",
        "notes" : [
                {
                        "id" : "ePce6fBAHx9KeKjuM"
                }
        ]
}

edited Jan 25, 2016 at 17:00

answered Jan 25, 2016 at 16:07

Owain Williams

2,65321 silver badges24 bronze badges

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

4 Comments

user3142695 Over a year ago

That's quite nice. Think I have to keep the timestamp and order also for timestamp. How would I do a map reduce on client?

Owain Williams Over a year ago

There's a good explanation of Map-Reduce on the MongoDB Docs website docs.mongodb.org/manual/core/map-reduce

Owain Williams Over a year ago

I have found a better way to do it with the $slice projection. I have updated my answer. I think this correctly solves your issue. Thanks for accepting my answer ;-)

Owain Williams Over a year ago

If you need other items included in the output, just add them to the projection, e.g. "notes.message": 1, you can also just exclude the items you don't want, e.g. Collection.find({},{"notes.message": -1}) will return everything except the message in each element of the notes array. You can read more about this in the db.collection.find() reference in the MongoDB Docs: docs.mongodb.org/manual/reference/method/db.collection.find

Collectives™ on Stack Overflow

MongoDB: Getting document and only one object of an array

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related