MongoDB aggregate within daily grouping

Question

I have some docs in mongo that looks something like this: ``` { _id : ObjectId("..."), "make" : "Nissan", .. }, { _id : ObjectId("..."), "make" : "Nissan", "saleDate" : ISODate("2013-04-10T12:39:50.676Z"), .. } ``` Ideally, I'd like to be able to count, by make, the number of vehicles sold per day. I'd then like to view either today, or a window such as today through the last seven days. I was able to accomplish the daily view with some ugly code ``` db.inventory.aggregate( { $match : { "saleDate" : { $gte: ISODate("2013-04-10T00:00:00.000Z"), $lt: ISODate("2013-04-11T00:00:00.000Z") } } } , { $group : { _id : { make : "$make", saleDayOfMonth : { $dayOfMonth : "$saleDate" } }, cnt : { $sum : 1 } } } ) ``` Which then yields the results ``` { "result" : [ { "_id" : { "make" : "Nissan", "saleDayOfMonth" : 10 }, "cnt" : 2 }, { "_id" : { "make" : "Toyota", "saleDayOfMonth" : 10 }, "cnt" : 4 }, ], "ok" : 1 } ``` So that is ok, but I would much prefer to not have to change the two datetime v…

Accepted Answer

In Mongo 2.8 RC2 there is a new data aggregation operator: $dateToString which can be used to group by a day and simply have a "YYYY-MM-DD" in the result:

Example from the documentation:

```
db.sales.aggregate(
  [
     {
         $project: {
                yearMonthDay: { $dateToString: { format: "%Y-%m-%d", date: "$date" } },
                time: { $dateToString: { format: "%H:%M:%S:%L", date: "$date" } }
         }
     }
  ]
)
```

will result in:

```
{ "_id" : 1, "yearMonthDay" : "2014-01-01", "time" : "08:15:39:736" }
```

MongoDB aggregate within daily grouping

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