Find distinct documents with max value of a field in mongodb

问题

I have thousands of documents in MongoDB with some of the sample as below:

{"title":"Foo", "hash": "1234567890abcedf", "num_sold": 49, 
"created": "2013-03-09 00:00:00"}

{"title":"Bar", "hash": "1234567890abcedf", "num_sold": 55, 
"created": "2013-03-11 00:00:00"}

{"title":"Baz", "hash": "1234567890abcedf", "num_sold": 55,
 "created": "2013-03-10 00:00:00"}

{"title":"Spam", "hash": "abcedef1234567890", "num_sold": 20,
 "created": "2013-03-11 00:00:00"}

{"title":"Eggs", "hash": "abc1234567890def", "num_sold": 20,
 "created": "2013-03-11 00:00:00"}

Is it possible to select all documents with distinct hash which has the max of num_sold and if there is more than one document with same num_sold, select the latest document from the created field.

I use PyMongo for the client.

回答1:

I am no Python expert so I will write this in JavaScript. You can do this with the aggregation framework using the $sort, $group and $first opreators:

db.col.aggregate([
    {$sort: {created:-1}},
    {$group: {_id: '$hash', num_sold: {$first: '$num_sold'}, _id_seen: {$first: '$_id'}}}
])

Essentially what I do is sort the incoming documents by their created date DESC and then I group on hash, concatenating two duplicate hashes and then I get the first result of the sorted group, which should be the newest document.

References:

• http://docs.mongodb.org/manual/reference/aggregation/first/#_S_first

来源：https://stackoverflow.com/questions/15334408/find-distinct-documents-with-max-value-of-a-field-in-mongodb