Aggregate MongoDB results by ObjectId date

Question

How can I aggregate my MongoDB results by ObjectId date. Example:

Default cursor results:

cursor = [
    {\'_id\': ObjectId(\'5220b974a61ad0000746c0d         


        

Answer 1

The Jira Ticket pointed out by llovett has been solved, so now you can use date operators like $isoWeek and $year to extract this information from an ObjectId.

Your aggregation would look something like this:

{
    "$project":
        {

            "_id": {
                "$dateFromParts" : {
                    "year": { "$year": "$_id"},
                    "month": { "$month": "$_id"},
                    "day": { "$dayOfMonth": "$_id"}
                }
            }
        }
}

Answer 2

There is no way to accomplish what you're asking with mongodb's aggregation framework, because there is no aggregation operator that can turn ObjectId's into something date-like (there is a JIRA ticket, though). You should be able to accomplish what you want using map-reduce, however:

// map function
function domap() {
    // turn ObjectId --> ISODate
    var date = this._id.getTimestamp();
    // format the date however you want
    var year = date.getFullYear();
    var month = date.getMonth();
    var day = date.getDate();

    // yields date string as key, entire document as value
    emit(year+"-"+month+"-"+day, this);
}

// reduce function
function doreduce(datestring, docs) {
    return {"date":datestring, "docs":docs};
}

Answer 3

So this doesn't answer my question directly, but I did find a better way to replace all that lambda nonsense above using Python's setdefault:

d = {}
for message in messages:
    key = message['_id'].generation_time.date()
    d.setdefault(key,[]).append(message)

Thanks to @raymondh for the hint in is PyCon talk:

Transforming Code into Beautiful, Idiomatic Python