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

I think I figured it out the following aggregate query. I would be interested in if there is a better way of doing this.

db.friends.aggregate([
    { $unwind: '$friends'},
    { $project: {
        _id: 1,
        name: 1,
        buddy: {
            nickName: '$friends.name',
            age: '$friends.age',
        }
    }},
    { $group: {        
        _id: '$_id',
        name: {'$first': '$name'},
        buddies: {"$push": "$buddy"}
    }}
]);

Below I describe the response of each operator ($unwind, $project, $group)

The $unwind would return something like this. It breaks out each Embedded Document into its own document. So where we had 1 document with 2 friends sub document, now we have two document. Both documents will have the same id and name fields but the friends sub document will be different:

{
    "_id" : ObjectId("539c80d43cb9fe99d183a5f7"),
    "name" : "Matthew",
    "friends" : {"name" : "slim", "age" : "32"}
}, {
    "_id" : : ObjectId("539c80d43cb9fe99d183a5f7"),,
    "name" : "Matthew",
    "friends" : {"name" : "buba", "age" : "36"}
}

The $project would return something like this. Here I'm creating a new Embedded Document called buddy and notice that I assigning "$friends.name" to "nickName":

{
    "_id" : ObjectId("539c80d43cb9fe99d183a5f7"),
    "name" : "Matthew",
    "buddy" : {"nickName" : "slim","age" : "32"}
}, {
    "_id" : : ObjectId("539c80d43cb9fe99d183a5f7"),,
    "name" : "Matthew",
    "friends" : {"nickName" : "buba","age" : "36"}
}

And then the $group would do something like this. Here I'm just group everything back together, but setting the array name to "buddies":

{
    "_id" : ObjectId("539c80d43cb9fe99d183a5f7"),
    "name" : "Matthew",
    "buddies" : [
        {"nickName" : "slim","age" : "32"},
        {"nickName" : "buba","age" : "36"}
    ]
}

This seems to work, but if the friends array had duplicates, this process would remove the duplicates. This can be solved if there is a unique id for each friend.