Mongodb mongo查询中的减法不起作用？

我的收藏如下：

{
"_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
            }
        ]
    }
   ]
  }

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);

但下一行仍然是6除以3，而不是2：

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

最后一行中的count似乎仍然引用count的旧值，而不是更新的值

有人能帮我吗

更新：

---

我自己认为，如果我先计算减法，然后再进行除法，它会工作，但我不知道如何进行？

您需要对

$project

对象进行轻微修改。您需要使用从

count

中减去

1

得到的对象，而不是使用

count

的上一个值

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));

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));

上述代码适用于

记录>=2的组。如果有一个组只有一条记录，则减法后的计数将为零，从而导致除以零的错误

因此，您可以修改代码，以包含一个，检查减法后的计数是否为
0
，如果是，则将其默认为
1
，否则保留
count
的减去值

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));

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));

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