Aggregate MongoDB results by ObjectId date - mongodb

Question

How can I aggregate my MongoDB results by ObjectId date. Example:
Default cursor results:
cursor = [
{'_id': ObjectId('5220b974a61ad0000746c0d0'),'content': 'Foo'},
{'_id': ObjectId('521f541d4ce02a000752763a'),'content': 'Bar'},
{'_id': ObjectId('521ef350d24a9b00077090a5'),'content': 'Baz'},
]
Projected results:
projected_cursor = [
{'2013-09-08':
{'_id': ObjectId('5220b974a61ad0000746c0d0'),'content': 'Foo'},
{'_id': ObjectId('521f541d4ce02a000752763a'),'content': 'Bar'}
},
{'2013-09-07':
{'_id': ObjectId('521ef350d24a9b00077090a5'),'content': 'Baz'}
}
]
This is what I'm currently using in PyMongo to achieve these results, but it's messy and I'd like to see how I can do it using MongoDB's aggregation framework (or even MapReduce):
cursor = db.find({}, limit=10).sort("_id", pymongo.DESCENDING)
messages = [x for x in cursor]
this_date = lambda x: x['_id'].generation_time.date()
dates = set([this_date(message) for message in messages])
dates_dict = {date: [m for m in messages if this_date(m) == date] for date in dates}
And yes, I know that the easiest way would be to simply add a new date field to each record then aggregate by that, but that's not what I want to do right now.
Thanks!

Answer 1

Update: There is a built in way to do this now, see https://stackoverflow.com/a/51766657/295687
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 2

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 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