Aggregate MongoDB Results by ObjectId Date

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 use in PyMongo to achieve these results, but it is messy, and I would like to see how I can do this using the MongoDB (or even MapReduce) aggregation structure:

 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 and then fill it out, but that’s not what I want to do right now.

Thanks!

Answer 1

So, this does not answer my question directly, but I found a better way to replace all this lambda nonsense above using Python setdefault :

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

Thanks @raymondh for the tip in PyCon:

Convert code to beautiful, idiomatic Python

Answer 2

It is not possible to fulfill what you are asking for with mongodb's aggregation structure, since there is no aggregation operator that can turn ObjectId into something dated (there is a JIRA ticket ). 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}; }