MongoDB $project embedded document to root level

Question

Using the aggregate pipeline, I am trying to project an embedded document to the root level WITHOUT projecting each field individually.

For example, I want to project name from this collection to the root level:

[
    {
        _id: "1",
        name: {
            firstName: "John",
            lastname: "Peters"
        }
    },
    {
        _id: "2",
        name: {
            firstName: "Mary",
            lastname: "Jones"
        }
    }
]

This is what I am looking for:

[
    {
        firstName: "John",
        lastname: "Peters"
    },
    {
        firstName: "Mary",
        lastname: "Jones"
    }
]

Is there a way to do this without projecting each field individually? I don't want to have to do this:

db.collection.aggregate(
    [
        {
            $project : {
                "_id" : 0,
                "firstName" : "$name.firstName",
                "lastName" : "$name.lastName"
            }
        }
    ]

Answer 1

MongoDB 3.4 has the new stage in aggregation pipeline - $replaceRoot, which does exactly what was asked.

https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/

Answer 2

Here is the solution which uses JavaScript variable.

# Set Object for what to project
var projectWhat = {'_id' : 0};

# Fill Object with keys
Object.keys(db.coll.findOne().name).forEach(function(x){
    projectWhat[x] = "$name." + x;
});

# Do Aggregate
db.coll.aggregate([{$project : projectWhat}])

And the output will be

{ "firstName" : "John", "lastname" : "Peters" }
{ "firstName" : "Mary", "lastname" : "Jones" }

Hope this helps.