Mongoose Query: Calculate the number of elements in an array

Question

Mongoose Query: Calculate the number of elements in an array

My Schema contains an array of ObjectIds:

const userSchema = new Schema({
    ...
    article: [{
        type: mongoose.Schema.Types.ObjectId,
    }],
    ...
},

https://i.sstatic.net/RcXTF.png

In the example above, I want to count the elements in the array, which should be 10.

I attempted the following method, but it didn't work for me. The req.query.id represents the _id of the user and is used to filter the specific user with a matching article array.

const userData = User.aggregate(
    [
        {
            $match: {_id: id}
        },
        {
            $project: {article: {$size: '$article'}}
        },
    ]
)
console.log(res.json(userData));

The console.log(article.length) currently returns 0. How can I achieve this? Is using the aggregate function the correct approach, or is there a better way to count elements in an array?

javascript arrays mongodb mongoose

Answer 1

Answer №1

Consider the benefits of utilizing aggregate even when an array of ids is already associated with the user object.

To incorporate articles as a reference field:

const {Schema} = mongoose.Schema;
const {Types} = Schema;

const userSchema = new Schema({
    ...
    article: {
      type: [Types.ObjectId],
      ref: 'Article',
      index: true,
    },
    ...
});

// include a virtual property if desired
userSchema.virtual('articleCount').get(function () {
  return this.article.length;
});

Retrieve articles using populate method:

const user = await User.findById(req.query.id).populate('articles');

console.log(user.article.length);

Alternatively, handle arrays of ids without populating:

const user = await User.findById(req.query.id);

console.log(user.article.length);

Utilize the virtual field for easy access:

const user = await User.findById(req.query.id);

console.log(user.articleCount);

It's worth considering using aggregate for complex post-filter logic that essentially involves aggregation. For instance, when you need to process a result set on the database side to obtain specific information efficiently, such as extracting users based on articles added on a particular day and grouping them by hour.

Answer 2

Consider the benefits of utilizing aggregate even when an array of ids is already associated with the user object.

To incorporate articles as a reference field:

const {Schema} = mongoose.Schema;
const {Types} = Schema;

const userSchema = new Schema({
    ...
    article: {
      type: [Types.ObjectId],
      ref: 'Article',
      index: true,
    },
    ...
});

// include a virtual property if desired
userSchema.virtual('articleCount').get(function () {
  return this.article.length;
});

Retrieve articles using populate method:

const user = await User.findById(req.query.id).populate('articles');

console.log(user.article.length);

Alternatively, handle arrays of ids without populating:

const user = await User.findById(req.query.id);

console.log(user.article.length);

Utilize the virtual field for easy access:

const user = await User.findById(req.query.id);

console.log(user.articleCount);

It's worth considering using aggregate for complex post-filter logic that essentially involves aggregation. For instance, when you need to process a result set on the database side to obtain specific information efficiently, such as extracting users based on articles added on a particular day and grouping them by hour.

Mongoose Query: Calculate the number of elements in an array

Answer №1

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