I am trying to project two elements of an array I got after joining two documents using $lookup. When I use the dot notation to access the array elements as below -

db.departments.aggregate([{
    $lookup: {
        from: 'employees',
        localField: 'dep_id',
        foreignField: 'department',
        as: 'emps'
    }
}, {
    $project: {
        _id: 0,
        emp_id: 1,
        salary: 1,
        emp_name: '$emps.name',
        username: '$emps.username'
    }
}])

I get the following result -

emp_id:910579
salary:100000
emp_name:Array
    0:"Stephen Wolf"
username:Array
    0:"StepWolf"

I want the result as follows -

emp_id:910579
salary:100000
emp_name:"Stephen Wolf"
username:"StepWolf"

Does anybody have any suggestions? I want something that can convert array element to attribute, similar to the ObjectToArray function.

CodePudding user response：

Query1

• the bellow does field : [m1 m2 m2] => field : m1
• takes the first member and makes it the value of the field

Test code here

aggregate(
[{"$set": 
   {"emp_name": {"$arrayElemAt": ["$emp_name", 0]},
    "username": {"$arrayElemAt": ["$username", 0]}}}])

Query2

• If you have MongoDB >=5 you can use $first also

Test code here

aggregate(
[{"$set": 
   {"emp_name": {"$first": "$emp_name"},
    "username": {"$first": "$username"}}}])

In your case use query1 or query2 inside the project you already have, like :

{
    $project: {
        _id: 0,
        emp_id: 1,
        salary: 1,
        "emp_name": {"$first": "$emp_name"},
        "username": {"$first": "$username"}
    }
}