在mongo查詢中減法不起作用？

Question

我有一個集合如下：

{ 
"_id" : ObjectId("5491d65bf315c2726a19ffe0"), 
"tweetID" : NumberLong(535063274220687360), 
"tweetText" : "19 RT Toronto @SunNewsNetwork: WATCH: When it comes to taxes, regulations, and economic freedom, is Canada more \"American\" than America? http://t.co/D?", 
"retweetCount" : 1, 
"source" : "<a href=\"http://twitter.com\" rel=\"nofollow\">Twitter Web Client</a>", 
"Date" : ISODate("2014-11-19T04:00:00.000Z"), 
"Added" : ISODate("2014-11-19T04:00:00.000Z"), 
"tweetLat" : 0, 
"tweetLon" : 0, 
"url" : "http://t.co/DH0xj0YBwD ", 
"sentiment" : 18, 
"quality" : 0.4, 
"intensity" : 10, 
"happiness" : 0, 
"calmness" : 0, 
"kindness" : 0, 
"sureness" : 0, 
"Hashtags" : [ 
    "harp", 
    "nknkn" 
], 
"authorID" : NumberLong(49067869), 
"authorName" : "Fran Walker", 
"authorFollowers" : 93, 
"authorFollowing" : 133, 
"authorFavourites" : 50, 
"authorTweets" : 13667, 
"authorVerified" : false, 
"screenName" : "snickeringcrow", 
"profileImageURL" : "http://pbs.twimg.com/profile_images/2180546952/smilinkitty.asp_-_Copy_normal.jpg", 
"profileLocation" : "", 
"timezone" : "Eastern Time (US & Canada)", 
"gender" : "M", 
"Entities" : [ 
    { 
     "id" : 6, 
     "name" : "Harper, Stephen", 
     "frequency" : 0, 
     "partyId" : 6 
    } 
], 
"Topics" : [ 
    { 
     "id" : 8, 
     "name" : "Employment", 
     "frequency" : 1, 
     "Subtopics" : [ 
      { 
       "id" : 34, 
       "name" : "Economic", 
       "frequency" : 1 
      } 
     ] 
    }, 
    { 
     "id" : 11, 
     "name" : "Economy", 
     "frequency" : 1, 
     "Subtopics" : [ 
      { 
       "id" : 43, 
       "name" : "Economic", 
       "frequency" : 1 
      } 
     ] 
    } 
    ] 
    } 

，我試圖通過日期得到組，並得到情緒的總和爲每個組由（每組數項的-1）劃分。正如你看到的，因爲那-1我不能使用蒙戈的AVG函數，所以我必須做手工如下：

DBCollection collectionG; 
    collectionG = db.getCollection("TweetCachedCollection"); 
    ArrayList<EntityEpochData> results = new ArrayList<EntityEpochData>(); 
    List<DBObject> stages = new ArrayList<DBObject>(); 
    ArrayList<DBObject> andArray = null; 
    DBObject groupFields = new BasicDBObject("_id", "$Added"); 
    groupFields.put("value", 
      new BasicDBObject("$sum", "$" + sType.toLowerCase())); 
    groupFields.put("count", new BasicDBObject("$sum", 1)); 

    DBObject groupBy = new BasicDBObject("$group", groupFields); 
    stages.add(groupBy); 
    DBObject project = new BasicDBObject("_id", 0); 
    project.put("count", new BasicDBObject("$subtract", new Object[] { 
      "$count", 1 })); 
    project.put("value", new BasicDBObject("$divide", new Object[] { 
      "$value", "$count" })); 
    project.put("Date", "$_id"); 
    stages.add(new BasicDBObject("$project", project)); 

    DBObject sort = new BasicDBObject("$sort", new BasicDBObject("Date", 1)); 
    stages.add(sort); 
    AggregationOutput output = collectionG.aggregate(stages); 

現在一切正常，除了：

可以說計數爲3，但是如果我加上它，我預計計數的數量是2，它是在減法之後，但是當涉及下一個計劃仍然計數的行時，指的是3.

有關更多解釋，例如，如果sum爲6並且計數爲3我想sum /（count-1）返回2，但它返回3 !!!! 如此看來，這行返回2：

project.put("count",new BasicDBObject("$subtract", new Object[] {"$count", 1 })); 

但下一行還通過3個而不是2分6：

project.put("value", new BasicDBObject("$divide", new Object[] { 
      "$value", "$count" })); 

看來，在最後一行數仍指舊值的計數，而不是更新一個...

任何人都可以幫助我嗎？

更新：

我自己想，如果我的舞臺減法，然後再去做工作分工，但我不知道該怎麼辦呢？

Answer 1

您需要對您的$project對象稍作修改。您需要使用通過從count減去1而獲得的對象，而不是使用之前的值count。

DBObject project = new BasicDBObject("_id", 0); 
DBObject countAfterSubtraction = new BasicDBObject("$subtract", 
            new Object[] {"$count", 1}); 
DBObject value = new BasicDBObject("$divide", 
          new Object[] {"$value",countAfterSubtraction}); 
project.put("value", value); 
project.put("Date", "$_id"); 
stages.add(new BasicDBObject("$project", project)); 

上述代碼適用於擁有records >= 2的羣組。如果只有一個組只有一個記錄，減法後的計數將爲零，導致除以0錯誤。

所以，你可以修改你的代碼，包括$cond，以檢查是否減法後的計數0，如果是的話，就默認爲1，否則保持count減去值。

DBObject project = new BasicDBObject("_id", 0); 
DBObject countAfterSubtraction = new BasicDBObject("$subtract", 
             new Object[] {"$count", 1}); 
DBObject eq = new BasicDBObject("$eq", 
         new Object[]{countAfterSubtraction,0}); 
DBObject cond = new BasicDBObject("$cond", 
         new Object[]{eq,1,countAfterSubtraction}); 
DBObject value = new BasicDBObject("$divide", 
           new Object[] {"$value",cond}); 
project.put("value", value); 
project.put("Date", "$_id"); 
stages.add(new BasicDBObject("$project", project));