Mongoose return array $size

Question

I have a document :

{
    _id: 98556a665626a95a655a,
    first_name: 'Panda',
    last_name: 'Panda,
    notifications: [{
        _id: '',
        // ...
    }]
}

I want to return the following response :

{
   _id: 98556a665626a95a655a,
   first_name: 'Panda',
   last_name: 'Panda',
   notifications: 2
}

The problem is about notifications count field,

I used Mongoose NodeJS package and I tried the following :

UserDBModel.findOne({_id: uid}, {notifications: {$size: '$notifications'}}, function(err, user){ });

But it seems to not work. Someone can help me ? :)

Thanks in advance.

the $size operator will only return the fields that matches the size specified when used like you used it (It expects a number). If you want it to return the number of items in the array then you must you use it in an aggregration. Here's the link. If not, try using the .length property of an array like suggested in the answers. — mugabits
– mugabits, Commented Dec 21, 2015 at 17:17

weiyin · Accepted Answer · 2015-12-21 17:30:39Z

4

Use aggregate with a project pipeline operator.

UserDBModel.aggregate()
    .match({_id: uid})
    .project({
        first_name: 1,
        last_name: 1,
        notifications: {$size:"$notifications"}
    })
    .exec(function(err, notifications) {
        // notifications will be an array, you probably want notifications[0]
    });

Note that you will have to explicitly specify the fields for the project operator.

edited Dec 21, 2015 at 17:30

answered Dec 21, 2015 at 17:21

weiyin

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2 Comments

Nïrio Over a year ago

Thanks it works ! Just need to add mongoose.Types.ObjectId(uid) instead of uid.

Ahmed Bargady Over a year ago

Great man, worked perfectly for me, THANKS

CodePhobia · Accepted Answer · 2015-12-21 17:06:48Z

2

Maybe, you could do something like this in your node.

UserDBModel.findOne({ '_id': uid }, 'notifications', function (err, notifications) {
  if (err) return handleError(err);

 console.log(notifications.length);
})

Since you are using JS anyways maybe use its power! :)

answered Dec 21, 2015 at 17:06

CodePhobia

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2 Comments

Nïrio Over a year ago

Yes but I want to avoid to use memory with notifications array content when I just need to count them.

CodePhobia Over a year ago

Okay! If you expect the notification arrays to be huge, then thats the way to go, Mongo will do the heavy lifting for you. But mind you that having boundlessly growing array within a collection in mongodb is a bad design. :)

ZiggarDude · Accepted Answer · 2019-06-03 15:56:53Z

2

What I found to work for me is creating a mongoose virtual attribute.

Schema.virtual('notifications_count').get(function() {
  if (this.notifications) {
    return this.notifications.length;
  }
});

answered Jun 3, 2019 at 15:56

ZiggarDude

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1 Comment

Ahmed Bargady Over a year ago

But how can we get that value in the res ?

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