Aggregate/project sub-document as top-level document in mongo

Question

After a few aggregation steps (pipeline steps) in one of my collections, I\'m ending up with the following result:

{
    \"_id\" : ObjectId(\"574e7722bffe901713d         


        

Answer 1

When you have many, many fields in the sub-document and occasionally it is updated with new fields, then projection is not a viable option. Fortunately, since 3.4, MongoDB has a new operator called $replaceRoot.

All you have to do is add a new stage at the end of your pipeline.

db.getCollection('sample').aggregate([
    {
        $replaceRoot: {newRoot: "$command"}
    },
    {
        $project: {score: 0 } //exclude score field
    }
])

This would give you the desired output.

Note that in case of aggregation (especially after a $group stage) the 'command' document could be an array and could contain multiple documents. In this case you need to $unwind the array first to be able to use $replaceRoot.