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I have a collection of errors, so that every error carries a date field. How can I aggregate/count/group the errors by DAY only (i.e. exclude the time of the day)? I guess, some smart projection should be applied.
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$project − Used to select some specific fields from a collection. $match − This is a filtering operation and thus this can reduce the amount of documents that are given as input to the next stage.
Definition. Evaluates a boolean and returns the opposite boolean value; i.e. when passed an expression that evaluates to true , $not returns false ; when passed an expression that evaluates to false , $not returns true .
$unwind treats the sizes field as a single element array if: the field is present, the value is not null, and. the value is not an empty array.
You can do this by using the following aggregation operators:
This gives you the error count for each date:
db.errors.aggregate( { $group : { _id: { year : { $year : "$date" }, month : { $month : "$date" }, day : { $dayOfMonth : "$date" }, }, count: { $sum: 1 } }} ); This example assumes that the date field in your error documents is date and of type BSON Date. There is also a Timestamp type in MongoDB, but use of this type is explicitely discouraged by the documentation:
Note: The BSON Timestamp type is for internal MongoDB use. For most cases, in application development, you will want to use the BSON date type. See Date for more information.
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