Convert ISO date to yyyy-mm-dd format

Question

Given collection(#name: users) Structure:

{
"_id" : ObjectId("57653dcc533304a40ac504fc"),
"username" : "XYZ",
"followers" : [ 
    {
        "count" : 31,
        "ts" : ISODate("2016-06-17T18:30:00.996Z")
    }, 
    {
        "count" : 31,
        "ts" : ISODate("2016-06-18T18:30:00.288Z")
    }
]
}

I want to query this collection based on username field, and ts to be returned in 'yyyy-mm-dd' format. Expected Output:

{
"_id" : ObjectId("57653dcc533304a40ac504fc"),
"username" : "XYZ",
"followers" : [ 
    {
        "count" : 31,
        "date" : "2016-06-17"
    }, 
    {
        "count" : 31,
        "date" : "2016-06-18"
    }
]
}

I have tried something like this:

db.users.aggregate([
{$match:{"username":"xyz"}},
{$project:{ "followers":{"count":1,
        "date":"$followers.ts.toISOString().slice(0,10).replace(/-/g,'-')"
          }}
}
])

But it doesn't seems to be working. Can anyone please help? Thanks much.

Solution

Consider running an aggregation pipeline that will allow you to flatten the data list first, project the new field using the $dateToString operator, then regroup the flattened docs to get your desired result.

The above can be shown in three distinct pipelines:

db.users.aggregate([
    { "$match": { "username": "xyz" } },
    { "$unwind": "$followers" },
    {
        "$project": {
            "username": 1,
            "count": "$followers.count",
            "date": { "$dateToString": { "format": "%Y-%m-%d", "date": "$followers.ts" } }
        }
    },
    {
        "$group": {
            "_id": "$_id",
            "username": { "$first": "$username" },
            "followers": { "$push": {
                "count": "$count",
                "date": "$date"
            }}
        }
    }
])

---

With MongoDB 3.4 and newer, you can use the new $addFields pipeline step together with $map to create the array field without the need to unwind and group:

db.users.aggregate([
    { "$match": { "username": "xyz" } },    
    {
        "$addFields": {
            "followers": { 
                "$map": { 
                    "input": "$followers", 
                    "as": "follower",
                    "in": { 
                        "count": "$$follower.count", 
                        "date": { 
                            "$dateToString": { 
                                "format": "%Y-%m-%d", 
                                "date": "$$follower.ts" 
                            }
                        } 
                    } 
                } 
            }
        }
    }
])