如何获取具有pymongo中mapreduce字段的最大值的文档

Question

如何在pymongo中找到具有map-reduce的最大uid字段的文档？

我尝试了以下方法，但它打印出空白：

from pymongo import Connection
from bson.code import Code
db = Connection().map_reduce_example
db.things.insert({  
    "_id" : "50f5fe8916174763f6217994", 
    "deu" : "Wie Sie sicher aus der Presse und dem Fernsehen wissen, gab es in Sri  Lanka mehrere Bombenexplosionen mit zahlreichen Toten.n", 
    "uid" : 13, 
    "eng" : "You will be aware from the press and television that there have been a  number of bomb explosions and killings in Sri Lanka." 
})
db.things.insert({  
    "_id" : "50f5fe8916234y0w3fvhv234", 
    "deu" : "Ich bin schwanger foo bar.n", 
    "uid" : 14, 
    "eng" : "I am pregnant foobar." 
})
db.things.insert({  
    "_id" : "50f5fe8916234y0w3fvhv234", 
    "deu" : "barbar schwarz sheep, haben sie any foon", 
    "uid" : 14, 
    "eng" : "barbar black sheep, have you any foo" 
})
m = Code("function () {emit(this.uid,{'uid':this.uid,'eng':this.eng})}")
r = Code("function (key, values) {var total = 0;for (var i = 0; i < values.length; i++) {total += values[i];}return total;}")

result = db.things.inline_map_reduce(m, r)
for r in result:
    print 

如下所示的示例文档：

{ 
    "_id" : ObjectId("50f5fe8916174763f6217994"), 
    "deu" : "Wie Sie sicher aus der Presse und dem Fernsehen wissen, gab es mehrere Bombenexplosionen mit zahlreichen Toten.n", 
    "uid" : 13, 
    "eng" : "You will be aware from the press and television that there have been a 
             number of bomb explosions and killings." 
}

Answer 1

您可以使用

find_one通过对该字段进行降序排序来查找具有最大uid的文档：

db.things.find_one(sort=[("uid", -1)])

或使用定义的常量：

db.things.find_one(sort=[("uid", pymongo.DESCENDING)])