如何使用pymongo中的map-reduce爲字段獲取最大值的文檔？

Question

如何使用pymongo中的map-reduce找到最大uid字段的文檔？

我曾嘗試以下，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 foo\n", 
    "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 

，看起來像這些示例文件：

{ 
    "_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找到與最大uid由DOC在該字段下降排序：

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

或使用定義的常量：

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