MongoDB aggregation framework sort by length of array

Question

Given the following data set:

{ "_id" : ObjectId("510458b188ce1d16e616129b"), "codes" : [ "oxtbyr", "xstute" ], "name" : "Ciao Mambo", "permalink" : "ciaomambo", "visits" : 1 }
{ "_id" : ObjectId("510458b188ce1d16e6161296"), "codes" : [ "zpngwh", "odszfy", "vbvlgr" ], "name" : "Anthony's at Spokane Falls", "permalink" : "anthonysatspokanefalls", "visits" : 0 }

How can I convert this python/pymongo sort into something that will work with the MongoDB aggregation framework? I'm sorting results based on the number of codes within the codes array.

z = [(x['name'], len(x['codes'])) for x in restaurants]
sorted_by_second = sorted(z, key=lambda tup: tup[1], reverse=True)
for x in sorted_by_second:
    print x[0], x[1]

This works in python, I just want to know how to accomplish the same goal on the MongoDB query end of things.

Maxim · Accepted Answer · 2013-01-29 00:20:41Z

3

> db.z.aggregate({ $unwind:'$codes'}, 
                 { $group : {_id:'$_id', count:{$sum:1}}}, 
                 { $sort :{ count: 1}})

answered Jan 29, 2013 at 0:20

Maxim

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2 Comments

Dan Gayle Over a year ago

That at least gives me a start. I'd like to have the full results shown, but at least I'm in the right direction. Any clues about the performance of this vs. the application side?

A. Jesse Jiryu Davis Over a year ago

this might get closer: db.z.aggregate({ $unwind:'$codes'}, { $group : {_id:{name:'$name', permalink:'$permalink'}, count:{$sum:1}}}, {$sort:{count:-1}}) Performance? You'll have to test, but chances are your MongoDB server is IO-bound rather than CPU-bound, so spending some CPU doing the aggregation there, instead of filling the wire with documents for processing in Python, is probably a win.

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