mongoDB : Creating An ObjectId For Each New Child Added To The Array Field

Question

mongodb 2.1.4(The Node Driver)

I'm currently trying to create a new ObjectID for each message I insert into an array(the array being a subdocument).

I figure this way - All CRUD operations can easily be performed on each message in the array.

For Example:

The "threads" collection(Note- An ObjectId for each message)

{
    "_id": ObjectId("1234132413424123"), //A thread id
    messages:[
        {
            _id :ObjectId("134124412341234"),// A message id
            "message":"MongoDB is my friend"

        },
        {
            _id :ObjectId("534124412342377"),
            "message":"MongoDB is my friend too"

        },
        ...
    ]
},
{
    "_id": ObjectId("22341324134224234"),
    messages:[
        {
            _id :ObjectId("8341244123411235"),
            "message":"Something clever"

        },
        {
            _id :ObjectId("134124412342376"),
            "message":"blah blah blah"

        },
        ...
    ]
}

What I'm currently doing right now:

var query = {};

query["_id"] = new ObjectID(threadID);

var update = {$push: {}};    //I write the update object externally just for aesthetics

update.$push["messages"] = newMessage; 

var threadsCollection = db.collection('threads');
threadsCollection.findOneAndUpdate(query,update, function (err, result) {
    if (err) {
        console.log(err);
    } 
    db.close();
});

Problem:

Unlike "insert" for collections, an update with $push does not create a new ObjectId for each message added to the array.

Question:

Is there a standard way of creating an ObjectID during a $push into the child array? Or should we just manually create an ObjectID and add it to the child beforehand?

Answer 1

Not a mongodb expert but, if I understand you correctly, you wish the _id field of the subdocument to be inserted automatically.

I created threads db and then inserted the first message in the messages collection using the following command:

db.messages.insert({messages:[{_id:ObjectId(), message:"Message 1."}]}); 

Notice the _id:ObjectId() field. The result is as follow:

Now that I have an ObjectId(56...), I can update that particular record and insert more messages to it. And the command is as follow:

db.messages.update({"_id":ObjectId("56...")}, 
{$push:{messages:{_id:ObjectId(), message:"I am message 2."}}});

And the above would insert the new message in the collection. See below screenshots:

and finally after a few updates the collection looks as follow:

All the _id fields in the messages array are automatically generated.

It might not be a good idea to use _id fields for various reasons. Please Google for more details on whether to use _id as key for subdocuments or not.

I used MongoDB shell version 3.0.6 for the commands.

EDIT 28-01-2016 16:09

Since the above solution was based on MongoDB shell, I decided to do another test using Node.js driver for MongoDB. First of all, I declare ObjectID variable as follow

var ObjectID = require('mongodb').ObjectID;

I will use the ObjectID with document id of the thread to which the message should be inserted as follow in my update and findOneAndUpdate function calls

app.get('/insertNewMessage', function(req, res) {
    db.collection("messages").findOneAndUpdate({
        _id: new ObjectID('56aa3554e90911b64c36a424')
    }, {
        $push: {
            messages: {
                _id: new ObjectID(),
                message: "From NodeJS with <3 using findOneAndUpdate.Bye."
            }
        }
    }, function(err, result) {
        if (err)
            res.json(err);
        else
            res.json(result);
    });
});

Tested both and works just fine. See the screenshot below: