Data type conversion in MongoDB

Question

I have a collection called Document in MongoDB. Documents in this collection have a field called CreationDate stored in ISO date type. My task is to count the number of docu

Answer 1

FOR MONGO >= 3.0 (after ~2015)

In case some poor souls stumble across this question in 2017, like me: As of Mongo 3.0 there is now a dateToString operator available

which means if you have proper Date()'s you should be able to do simply:

db.Document.aggregate( 
    , {$project: {_id:1, CreationDate:1}
    , {$group: {
         _id : { $dateToString: { format: "%Y-%m-$d", date: "$CreationDate" } },
         cnt:{$sum:1}}}
    , {$sort:{'cnt':-1}}
);

For those of us who happen to have dates stored in non-date field (what joy!) you can create a new Date() object in the project step. In my case, the date was stored as a number of milliseconds (integer) and I add the number of milliseconds to the 0-Date.

db.Document.aggregate( 
    , {$project: {
         _id:1, 
         CreationDate: {"$add": [ new Date(0), "$CreatedOn" ]}
}
    , {$group: {
         _id : { $dateToString: { format: "%Y-%m-$d", date: "$CreationDate" } },
         cnt:{$sum:1}}}
    , {$sort:{'cnt':-1}}
);