Project an array with MongoDB

Question

I'm using MongoDB's aggregation pipeline, to get my documents in the form that I want. As the last step of aggregation, I use $project to put the documents into their final form.

But I'm having trouble projecting and array of sub-documents. Here is what I currently get from aggrgation:

{
  "_id": "581c8c3df1325f68ffd23386",
  "count": 14,
  "authors": [
    {
      "author": {
        "author": "57f246b9e01e6c6f08e1d99a",
        "post": "581c8c3df1325f68ffd23386"
      },
      "count": 13
    },
    {
      "author": {
        "author": "5824382511f16d0f3fd5aaf2",
        "post": "581c8c3df1325f68ffd23386"
      },
      "count": 1
    }
  ]
}

I want to $project the authors array so that the return would be this:

{
  "_id": "581c8c3df1325f68ffd23386",
  "count": 14,
  "authors": [
    {
      "_id": "57f246b9e01e6c6f08e1d99a",
      "count": 13
    },
    {
      "_id": "5824382511f16d0f3fd5aaf2",
      "count": 1
    }
  ]
}

How would I go about achieving that?

Answer 1

You can unwind the array and wind it u again after projecting. Something like this:

db.collectionName.aggregate([
{$unwind:'$authors'},
{$project:{_id:1,count:1,'author.id':'$authors.author.author','author.count':'$authors.count'}},
{$group:{_id:{_id:'$_id',count:'$count'},author:{$push:{id:'$author.id',count:'$author.count'}}}},
{$project:{_id:0,_id:'$_id._id',count:'$_id.count',author:1}}
])

the output for above will be:

{ 
    "_id" : "581c8c3df1325f68ffd23386", 
    "author" : [
        {
            "id" : "57f246b9e01e6c6f08e1d99a", 
            "count" : 13.0
        }, 
        {
            "id" : "5824382511f16d0f3fd5aaf2", 
            "count" : 1.0
        }
    ], 
    "count" : 14.0
}