mongo aggregation unwind example

mongo_aggregation_unwind.js

db.t4.insert({
    "_id" : ObjectId("512e28984815cbfcb21646a7"),
    "list" : [
        {
            "a" : ISODate("2012-12-19T06:01:17.171Z"),
            "b" : 5
        },
        {
            "a" : ISODate("2013-12-19T06:01:17.171Z"),
            "b" : 5
        },
        {
            "a" : ISODate("2014-12-19T06:01:17.171Z"),
            "b" : 5
        },
        {
            "a" : ISODate("2015-12-19T06:01:17.171Z"),
            "b" : 10
        },
        {
            "a" : ISODate("2016-12-19T06:01:17.171Z"),
            "b" : 5
        }
    ]
})

db.t4.aggregate(
    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $unwind: '$list'},
    { $match: {'list.a': {$gt: ISODate("2013-12-19T06:01:17.171Z")}}},
    { $group: {_id: '$_id', list: {$push: {"val":'$list.a',"b":"$list.b"}}}})