Set field in mongoose document to array length

Question

I have a Mongoose document (Mongoose 5.4.13, mongoDB 4.0.12):

var SkillSchema = new mongoose.Schema({
    skill: { type: String },
    count: { type: Number, default: 0 },
    associatedUsers: [{ type : mongoose.Schema.Types.ObjectId, ref: 'User' }]
});

That I update as follows:

var query = { skill: req.body.skill };
var update = { $addToSet: { associatedUsers: req.params.id } };
            
var options = { upsert: true, new: true, setDefaultsOnInsert: true };

await skillSchema.findOneAndUpdate(query, update, options);

During this update, I would like to also update count to be equal to the length of associatedUsers.

Ideally I want this to happen at the same time as updating the other fields (i.e not in a subsequent update), either via a pre-hook or within findOneAndUpdate.

I've tried using a pre hook after schema definition:

SkillSchema.pre('findOneAndUpdate', async function(){
    console.log("counting associated users");
    this.count = this.associatedUsers.length;
    next();
});

As well as using aggregate in my UPDATE route:

await skillSchema.aggregate([{ $project: { count: { $size: "$associatedUsers" } } } ])

But I can't get either to work.

Does anyone have any suggestions for how I could achieve this?

Did the answers below help you?

O'Dane Brissett
– O'Dane Brissett

2019-10-03 14:28:44 +00:00
Commented Oct 3, 2019 at 14:28 — O'Dane Brissett
– O'Dane Brissett, Commented Oct 3, 2019 at 14:28

s7vr · Accepted Answer · 2019-10-04 15:46:31Z

6

+50

You could use $set like this in 4.2 which supports aggregation pipeline in update.

The first $set stage calculates a associatedUsers based on the previous and new value. $setUnion to keep the distinct associatedUsers values.

The second $set stage calculates tally based on the associatedUsers calculated in the previous stage.$size to calculate the length of associatedUsers values.

var query = {skill: req.body.skill};
var update = [{ $set: { "associatedUsers":{"$setUnion":[{"$ifNull":["$associatedUsers",[]]}, [req.params.id]] }}}, {$set:{tally:{ $size: "$associatedUsers" }}}];
var options = { upsert: true, new: true, setDefaultsOnInsert: true };
await skillSchema.findOneAndUpdate(query, update, options)

If any argument resolves to a value of null or refers to a field that is missing, $setUnion returns null. So just needed to safeguard our operation with $ifNull

edited Oct 4, 2019 at 15:46

answered Oct 3, 2019 at 14:31

s7vr

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7 Comments

fugu Over a year ago

This looks like an excellent solution. I will test this tonight

sushant mehta Over a year ago

in case of upsert, it'll give error arguement to $size must be an array

s7vr Over a year ago

@sushantmehta Thanks. Updated answer to account for null values in the set operation. That is behavior for set operators not specially related to upsert.

fugu Over a year ago

This isn't working for me:

var update = [{ $set: { "associatedUsers":{"$setUnion":[{"$ifNull":["$associatedUsers",[] ]}, [req.params.id]] }}}, { $set: { count: { $size: "$associatedUsers" }}}];

Note: I've changed the schema field tally to count. Will update the question to reflect this

s7vr Over a year ago

Np. Sorry, expressive updates is a new feature in 4.2. It will not work with older version. You probably have to do in multiple queries.

|

JozeV · Accepted Answer · 2019-05-07 15:45:53Z

2

About tally and associatedUsers.length

// define your schema object
var schemaObj = {
  skill: { type: String },
  associatedUsers: { type: Array }
};

// get the length of users
var lengthOfAsUsers = schemaObj.associatedUsers.length;

// add tally to schema object and set default to the length of users
schemaObj.tally = { type: Number, default: lengthOfAsUsers };

// and pass your schema object to mongoose.Schema
var SkillSchema = new mongoose.Schema(schemaObj);

module.exports = SkillSchema;

EDIT you can update tally subsequently, but recommended solution would be to use this method https://mongoosejs.com/docs/populate.html

const id = "nameSomeId";

SkillSchema.find({ _id: id }).then(resp => {
  const tallyToUpdate = resp.associatedUsers.length;
  SkillSchema.findOneAndUpdate({ _id: id }, { tally: tallyToUpdate }).then(
    resp => {
      console.log(resp);
    }
  );
});

edited May 7, 2019 at 15:45

answered May 7, 2019 at 15:10

JozeV

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3 Comments

fugu Over a year ago

But is this dynamic? I.e will the tally increase as I add associatedUsers?

JozeV Over a year ago

Well, you can always update tally subsucvently.. I will edit my answer

fugu Over a year ago

sure, but this require I make two requests which is not efficient

sushant mehta · Accepted Answer · 2019-10-04 10:56:25Z

The solution I have will only work on mongodb v 4.2 as it has option to use aggregate in the update and will only need one query as:

skillSchemafindOneAndUpdate(


 {skill:"art"},
   [
     { $set: { 
       associatedUsers:{
         $cond:{
            if: {$gte: [{$indexOfArray: ["$associatedUsers", mongoose.Types.ObjectId(req.params.id)]}, 0]},
              then: "$associatedUsers",
              else: { $cond:{
                if: { $isArray: "$associatedUsers" },
                then: {$concatArrays:["$associatedUsers",[mongoose.Types.ObjectId(req.params.id)]]},
                else: [mongoose.Types.ObjectId(req.params.id)]
           }}
           }
      }}},
      {$set:{
        associatedUsers:"$associatedUsers",
        tally:{$size:"$associatedUsers"},
      }}
   ],
   {upsert:true,new:true}
)

ref: https://docs.mongodb.com/manual/reference/method/db.collection.update/#update-with-aggregation-pipeline

Kunal Mukherjee · Accepted Answer · 2019-05-09 14:57:28Z

0

The "Group" field does not appear in the schema. On MongoDB Shell, these codes will work.

However, Mongoose will also give an error because the schema is validated.

Is the "Group" field a dynamic field? I think the problem with the schema will be solved.

var mongoose = require("mongoose");

var SkillSchema = new mongoose.Schema({
    skill: { type: String },
    tally: { type: Number, default: 0 },
    associatedUsers: { type: Array },
    group: { type: Array }
 });

edited May 9, 2019 at 14:57

Kunal Mukherjee

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answered May 7, 2019 at 15:13

Rıdvan Can

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