Mongoose - get length of array in model

Question

I have this Mongoose schema:

var postSchema = mongoose.Schema({

    postId: {
        type: Number,
        unique: true
    },

    upvotes: [
        {
            type: Number,
            unique: true
        }
    ]

});

what the best query to use to get the length of the upvotes array? I don't believe I need to use aggregation because I only want to query for one model, just need the length of the upvotes array for a given model.

Really struggling to find this info online - everything I search for mentions the aggregation methodology which I don't believe I need.

Also, as a side note, the unique schema property of the upvotes array doesn't work, perhaps I am doing that wrong.

For the unique index part of your question, see stackoverflow.com/questions/15921700/… — JohnnyHK
– JohnnyHK, Commented Oct 31, 2015 at 14:32
thanks @johnnyHK, $addToSet is working for me - the only problem I have with it is that it doesnt throw an error when you try to add a duplicate value, it merely returns "not modified" - seems more consistent to return an error but idk — Alexander Mills
– Alexander Mills, Commented Oct 31, 2015 at 17:08

JohnnyHK · Accepted Answer · 2015-10-31 14:28:21Z

20

find results can only include content from the docs themselves¹, while aggregate can project new values that are derived from the doc's content (like an array's length). That's why you need to use aggregate for this, even though you're getting just a single doc.

Post.aggregate([{$match: {postId: 5}}, {$project: {upvotes: {$size: '$upvotes'}}}])

¹_{Single exception is the $meta projection operator to project a $text query result's score.}

answered Oct 31, 2015 at 14:28

JohnnyHK

313k70 gold badges632 silver badges479 bronze badges

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

2 Comments

Alexander Mills Over a year ago

I will try this but I swear I tried this last night and it didnt work

Kamalakannan J Over a year ago

But Aggregation return JSON objects instead of Model objects. So we can't do any operation in returned documents. Any other ways ?

Ryan Knell · Accepted Answer · 2016-04-22 04:16:01Z

8

I'm not normally a fan of caching values, but it might be an option (and after finding this stackoverflow answer is what I'm going to do for my use case) to calculate the length of the field when the record is updated in the pre('validate') hook. For example:

var schema = new mongoose.Schema({
    name: String,
    upvoteCount: Number,
    upvotes: [{}]
});

schema.pre('validate', function (next) {
  this.upvoteCount = this.upvotes.length
  next();
});

Just note that you need to do your updates the mongoose way by loading the object using find and then saving changes using object.save() - don't use findOneAndUpdate

answered Apr 22, 2016 at 4:16

Ryan Knell

6,3603 gold badges42 silver badges32 bronze badges

Comments

ram · Accepted Answer · 2021-03-14 09:27:59Z

3

postSchema.virtual('upvoteCount').get(function () {
    return this.upvotes.length
});

let doc = await Post.findById('foobar123')

doc.upvoteCount // length of upvotes

answered Mar 14, 2021 at 9:27

ram

6905 silver badges17 bronze badges

Comments

prasun · Accepted Answer · 2015-11-01 02:21:01Z

2

My suggestion would be to pull the entire upvotes fields data and use .length property of returned array in node.js code

//logic only, not a functional code
post.find( filterexpression, {upvote: 1}, function(err, res){
  console.log(res.upvotes.length); 
});

EDIT:

Other way of doing would be stored Javascript. You can query the upvote and count the same in mongodb side stored Javascript using .length

edited Nov 1, 2015 at 2:21

answered Oct 31, 2015 at 8:47

prasun

7,3799 gold badges47 silver badges62 bronze badges

2 Comments

Alexander Mills Over a year ago

I think ideally we would want to make mongo do the work and leave as much code out of Node.js as possible, but yes this might work

Alexander Mills Over a year ago

thanks - JohnnyHKs solution is by far the most superior one

Collectives™ on Stack Overflow

Mongoose - get length of array in model

4 Answers 4

2 Comments

Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related