在mongodb中查找具有字段最大值的不同文档 [英] Find distinct documents with max value of a field in mongodb

问题描述

我在MongoDB中有成千上万的文档，其中的一些示例如下:

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"}

是否可以选择所有具有最大num_sold的不同hash的文档，如果有多个具有相同num_sold的文档，请从created字段中选择最新的文档.

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.

我将PyMongo用于客户端.

I use PyMongo for the client.

推荐答案

我不是Python专家，所以我将用JavaScript编写.您可以使用$sort，$group和$first操作符，通过聚合框架来做到这一点:

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'}}}
])

基本上，我要做的是按照创建的日期DESC对传入的文档进行排序，然后对哈希进行分组，将两个重复的散列连接起来，然后得到排序后的组的第一个结果，该结果应该是最新的文档.

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.

参考文献:

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

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