get length of array field in mongoose _ Nodejs

Question

suppose we have a User model that contains an array of other User objects.

let UserSchema = mongoose.Schema({
   followers: [{
        type: mongoose.Schema.ObjectId,
        ref: 'User',
    }]
})

I need a count of this objectIds. the first solution is to get length of followers.

req.user.followers.length

but I think it's not relevant to get all the followers that contains many of objectIds. and in my query i dont need all of this objectIds. I tried to use virtuals but in virtuals I have many unnecessary pieces of stuff.I'm looking for the best and uncosted way for this type of situations.

Kai · Accepted Answer · 2018-11-06 10:04:33Z

3

because of misunderstanding your question, so I update my answer: you can use $size of mongo aggregate.

db.users.aggregate(
   [
      {
         $project: {
            id: 1,
            total_followers: { $size: "$followers" }
         }
      }
   ]
)

In case of you want to find any document with specific number of length (eg: 0), you can do this :

db.users.find({ followers: { $size: 0 } } )

edited Nov 6, 2018 at 10:04

answered Nov 6, 2018 at 9:04

Kai

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4 Comments

Babak Abadkheir Over a year ago

if I filter {'followers ' :0} .i cant get count of it?

Kai Over a year ago

@babakabadkheir I updated my answer :)! with aggregate, you can filter or do whatever on data there.

Babak Abadkheir Over a year ago

dude i need a relevant solution. if I use aggregate in everywhere for getting count its not work when i need to populate information of followers.if i have 100 user i must after aggregate use populate this ids .it means aggregate + 100 populate !!

Kai Over a year ago

I guess the only way is get req.user.followers.length, because your schema was designed for that way.

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