MongoDB $lookup result as object instead of array

Question

Table "user":

{
    "_id" : ObjectId("573b09e6322304d5e7c6256e"),
    "name" : "John",
    "age" : 30,
    "list" : [ 
        "userId1", 
        "userId2", 
        "userId3", 
        "userId2"
    ]
},
{
    "_id" : "userId1",
    "name" : "Derek",
    "age" : 34
},
{
    "_id" : "userId2",
    "name" : "Homer",
    "age" : 44
},
{
    "_id" : "userId3",
    "name" : "Bobby",
    "age" : 12
}

My query:

db.user.aggregate(
    [
        {
            $match: { "_id": ObjectId("573b09e6322304d5e7c6256e") }
        },
        {
            $lookup:
            {
                from: "user",
                localField: "list",
                foreignField: "_id",
                as: "listData"
            }
        }
    ]
)

I need to findOne user by id with joined list data as id/value object:

{
    "_id" : ObjectId("573b09e6322304d5e7c6256e"),
    "name" : "John",
    "age" : 30,
    "list" : [ 
        "userId1", 
        "userId2", 
        "userId3", 
        "userId2"
    ],
    "listData" : {
        "userId1": {
            "_id" : "userId1",
            "name" : "Derek",
            "age" : 34
        }, 
        "userId2": {
            "_id" : "userId2",
            "name" : "Homer",
            "age" : 44
        }, 
        "userId3": {
            "_id" : "userId3",
            "name" : "Bobby",
            "age" : 12
        }
    ]
}

But I got listData as array:

{
    "_id" : ObjectId("573b09e6322304d5e7c6256e"),
    "name" : "John",
    "age" : 30,
    "list" : [ 
        "userId1", 
        "userId2", 
        "userId3", 
        "userId2"
    ],
    "listData" : [ 
        {
            "_id" : "userId1",
            "name" : "Derek",
            "age" : 34
        }, 
        {
            "_id" : "userId2",
            "name" : "Homer",
            "age" : 44
        }, 
        {
            "_id" : "userId3",
            "name" : "Bobby",
            "age" : 12
        }
    ]
}

What i need to change to get array as object with keys as ids? db version v4.0.2

Answer 1

You can use $addFields to turn listData into an object. Before that you should use $map to transform an array into an array of k-v pairs and then you can use $arrayToObject to generate new keys dynamically, try:

db.col.aggregate([
    {
            $match: { "_id": ObjectId("573b09e6322304d5e7c6256e") }
    },
    {
        $lookup:
        {
            from: "user",
            localField: "list",
            foreignField: "_id",
            as: "listData"
        }
    },
    {
        $addFields: {
            listData: {
                $arrayToObject: {
                    $map: {
                        input: "$listData",
                        as: "user",
                        in: {
                            k: "$$user._id",
                            v: "$$user"
                        }
                    }
                }
            }
        }
    }
])