Can the MongoDB Aggregation framework combine $group out?

Question

Here is my query, I would like to combine $_id to YYYY-MM-DD? or any function like Mysql DATE() to convert DATETIME format to DATE format?

db.event.aggregate([
{
    $project: {
        "created": {$add: ["$created", 60*60*1000*8]},
    }
},    
{
    $group: {
        "_id": {
            "year": {"$year": "$created"},
            "month": {"$month": "$created"},
            "day": {"$dayOfMonth": "$created"}
        },
        "count": { $sum: 1 }
    }
}

])

Answer 1

You basically already are by using the date aggregation operators to split up the components into your compound _id key, and this is probably the best way to handle it. You can actually alter this though with the $substr operator and use of $concat:

db.event.aggregate([
    { "$project": {
            "created": {$add: ["$created", 60*60*1000*8]},
    }},    
    { "$group": {
        "_id": {
            "year": {"$year": "$created"},
            "month": {"$month": "$created"},
            "day": {"$dayOfMonth": "$created"}
        },
        "count": { "$sum": 1 }
    }},
    { "$project": {
        "_id": { "$concat": [
            { "$substr": [ "$_id.year", 0, 4 ] },
            "-",
            { "$cond": [
                { "$lte": [ "$_id.month", 9 ] },
                { "$concat": [ 
                    "0", 
                   { "$substr": [ "$_id.month", 0, 2 ] } 
                ]},
                { "$substr": [ "$_id.month", 0, 2 ] }
            ]},
            "-",
            { "$cond": [
                { "$lte": [ "$_id.day", 9 ] },
                { "$concat": [
                    "0",
                    { "$substr": [ "$_id.day", 0, 2 ] }
                ]},
                { "$substr": [ "$_id.day", 0, 2 ] }
            ]}
        ]},
        "count": 1
    }}
])

So there is a bit of coercion of the values from the date parts to strings there as well as padding out any values under two didgits with a leading 0 just like in a "YYYY-MM-DD" format.

Noting that it can be done, and has been able to be done for some time, but it is notably missing from the manual page description of the $substr operator.

Not to sure about your "date math" at the start there. I would say you would be better off using the aggregation operators and then working on the values that you wanted to adjust by, or if indeed it was something like a "timezone" correction, then again you would probably be better off processing that client side.