MongoDB Aggregation Query- Rename Fields Returned from Within Embedded Documents

Question

I\'m currently using aggregate operator to return documents that has an array of embedded (sub) documents. I want to rename the field name for the array and also rename fie

Answer 1

There are a couple of approaches to this, but it largely depends on your MongoDB version. More recent versions from 2.6 and upwards support the $map operator which you can use in $project to do what you want:

db.friend.aggregate([
    { "$project": {
        "name": 1,
        "buddies": {
            "$map": {
                "input": "$friends",
                "as": "el",
                "in": {
                    "nickName": "$$el.name",
                    "age": "$$el.age"
                }
            }
        }
    }}
])

In prior versions you would use $unwind to work with the array elements and re-construct via $group:

db.collection.aggregate([
    { "$unwind": "$friends" },
    { "$group": {
        "_id": "$_id",
        "name": { "$first": "$name" },
        "buddies": {
            "$push": {
                "nickName": "$friends.name",
                "age": "$friends.age"
            }
        }
    }}
])

With the first form being a little more efficient as you are not de-normalizing the array content and producing more documents in the pipeline to process.