How To Join Multiple Collections With $lookup In MongoDB

In this article, we will learn how to join multiple collections with $lookup.

$lookup function allows us to join multiple collections in the same database. the $lookup function (joined collection) will return the sub-array of matched data.

syntax:

$lookup: 
        {
            from: collection to join,
            localField: field from the execute query document,
            foreignField: field from the join collection document(match with localField),
            as:name of the output array
        }

from: specifies the collection to perform the join.

localField: specifies the field from the document whose field match to the other collection document

foreignField : specifies the field from the other document

as: specifies the name of the new array to add the result

–first, we need to create two collections:

  • schema:
//for employee collection 
var employeeSchema = new mongoose.Schema({
    name: {
        type: String,
    },
    email: {
        type: String,
    }
});

//for employeedata collection
var employeeDataSchema = new mongoose.Schema({
    empid: {
        type: mongoose.Schema.ObjectId,
    },
    address: {
        type: String,
    },
    phone: {
        type: Number,
    }
});


const empSchema = mongoose.model('employee', employeeSchema);
const empDataSchema = mongoose.model('employeedata', employeeDataSchema);
  •  insert data in employee collection:
//insert data in employee collection
exports.addEmp = async (req, res) => {
    let data = await new empSchema(req.body).save()
    res.status(200).send({
        isSuccess: true,
        data:data
    });
}


output:

  • insert data in employeedata collection
exports.add = async (req, res) => {
    let result = new empDataSchema(req.body).save()
    res.status(200).send({
        isSuccess: true,
        data:result
    });
}

output:

–join employee and employeedata collection using $lookup

exports.empData = async (req, res) => {
    const data = await empSchema.aggregate([
        {
            $lookup:
            {
                from: "employeedatas",
                localField: "_id",
                foreignField: "empid",
                as: "emp"
            }
        }
    ]);
    res.status(200).send({
        isSuccess: true,
        data: data
    });
}

output:

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