Get the Most Recent Item in a Group

query

{ "_id": 1, "color": "red", "date": ISODate("2013-05-10T00:00:00Z") }
{ "_id": 2, "color": "blue", "date": ISODate("2016-10-20T00:00:00Z") }
{ "_id": 3, "color": "green", "date": ISODate("2015-09-14T00:00:00Z") }
{ "_id": 4, "color": "green", "date": ISODate("2015-07-19T00:00:00Z") }
{ "_id": 5, "color": "red", "date": ISODate("2017-01-04T00:00:00Z") }
{ "_id": 6, "color": "blue", "date": ISODate("2014-03-06T00:00:00Z") }

result = db.books.aggregate([
    { $sort: { "date": -1 } },
    { $group: {
        _id: "$color",
        books: { $push: "$$ROOT" }
    }},
    { $replaceRoot: {
        newRoot: { $arrayElemAt: ["$books", 0] }
    }}
]);