MongoDb Query on array field of a nested object

Question

I'm trying to query all users that favorited an event. The event Id is stored in the users document in a nested object.

My schema looks like this

{
    email: {
      type: String,
      unique: true,
      required: [true, 'Email is required!'],
      trim: true,
      validate: {
        validator(email) {
          const emailRegex = /^[-a-z0-9%S_+]+(\.[-a-z0-9%S_+]+)*@(?:[a-z0-9-]{1,63}\.){1,125}[a-z]{2,63}$/i;
          return emailRegex.test(email);
        },
        message: '{VALUE} is not a valid email!',
      },
    },
    role: {
      type: String,
      enum: ['admin', 'user', 'planner'],
      default: 'user',
    },
    name: {
      type: String,
      trim: true,
    },
    username: {
      type: String,
      trim: true,
      unique: true,
    },
    password: {
      type: String,
      required: [true, 'Password is required!'],
      trim: true,
      minlength: [6, 'Password needs to be longer!'],
      validate: {
        validator(password) {
          return password.length >= 6 && password.match(/\d+/g);
        },
      },
    },
    picture: {type: String},
    favorites: {
      events: [
        {
          type: Schema.Types.ObjectId,
          ref: 'Event',
        },
      ],
    },
  }

How would I write this query?

I've tried all kinds of combinations with $elemMatch and normal queries aswell

Thanks @AnthonyWinzlet surprisingly that worked, the problem was I was passing a string to it instead of passing a object id when I tried it before! — Matheus Rezende
– Matheus Rezende, Commented Dec 17, 2018 at 1:37

Rom · Accepted Answer · 2018-12-17 01:37:24Z

$elemMatch should be used as an object, but in array favorites.events each element is String (ObjectID), so you should use $eq to match each element with String which is ID of event you want.

The solution is:

User.find({
    'favorites.events': {
        '$elemMatch': {
            '$eq': id
        }
    }
})

Document for $elemMatch here https://docs.mongodb.com/manual/reference/operator/query/elemMatch/#element-match

And document for $eq here https://docs.mongodb.com/manual/reference/operator/query/eq/

You should question with simple structure of Mongo. Example, this is my test for you problem

const Schema = mongoose.Schema;

const EventSchema = new Schema({
    title: String
})

const UserSchema = new Schema({
    username: String,
    favorites: {
        events: [{
            type: Schema.Types.ObjectId,
            ref: 'Event',
        }]
    }
});

const User = mongoose.model('User', UserSchema);
const Event = mongoose.model('Event', EventSchema)

app.get('/users/event/:id', (req, res) => {
    const {id} = req.params;
    console.log(id);
    User.find({
        'favorites.events': {
            '$elemMatch': {
                '$eq': id
            }
        }
    }).then(u => res.send(u)) })

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