Mongo DB复制问题，同时使用带限制的排序和聚合中的跳过 [英] Mongo DB duplication issue while using sorting with limit and skip in aggregation

问题描述

在通过使用跳过和限制进行排序来获取记录时遇到重复记录的问题:

Facing an issue of duplicate records while fetching record by sorting with skip and limit:

收集数据:

{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78b"), 
    "name" : "F", 
    "percentage" : 60.0, 
    "weightedFilter" : 2.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78c"), 
    "name" : "I", 
    "percentage" : 80.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78d"), 
    "name" : "J", 
    "percentage" : 80.0, 
    "weightedFilter" : 1.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78e"), 
    "name" : "A", 
    "percentage" : 100.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78f"), 
    "name" : "K", 
    "percentage" : 80.0, 
    "weightedFilter" : 1.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a790"), 
    "name" : "G", 
    "percentage" : 60.0, 
    "weightedFilter" : 2.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a791"), 
    "name" : "H", 
    "percentage" : 80.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a792"), 
    "name" : "B", 
    "percentage" : 100.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0

}

汇总查询1:

db.testing.aggregate([{$sort : { "like": -1 }},{$skip : 0},{$limit:4}]);

输出:

{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78d"), 
    "name" : "J", 
    "percentage" : 80.0, 
    "weightedFilter" : 1.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78b"), 
    "name" : "F", 
    "percentage" : 60.0, 
    "weightedFilter" : 2.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78c"), 
    "name" : "I", 
    "percentage" : 80.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78e"), 
    "name" : "A", 
    "percentage" : 100.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}

---

汇总查询2:

db.testing.aggregate([{$sort : { "like": -1 }},{$skip : 4},{$limit:4}]);

{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78b"), 
    "name" : "F", 
    "percentage" : 60.0, 
    "weightedFilter" : 2.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78c"), 
    "name" : "I", 
    "percentage" : 80.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a78e"), 
    "name" : "A", 
    "percentage" : 100.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}
{ 
    "_id" : ObjectId("594b507c9b9469ec9da6a792"), 
    "name" : "B", 
    "percentage" : 100.0, 
    "weightedFilter" : 0.0, 
    "like" : 1.0, 
    "attraction" : 1.0
}

结论:

更改跳过0-> 4时，获得了名称为 F，I，A

When changing skip 0->4, got duplicate record having name F,I,A

不明白为什么会这样吗?

Not getting why this happen?

推荐答案

根据您的集合数据，您正在按具有共同值的键进行排序.

As per your collection data you are sorting by key having common values.

在第一个聚合聚合中，您使用(skip，limit)=>(0,4)，在这种情况下，mongo从所有文档中按顺序对文档进行排序，并对结果进行排序.

In first Aggregation aggregation you are using (skip,limit) => (0,4) in this case mongo is sorting the documents in order from all the documents and the result is sorted.

在第二个聚合中，您再次使用(skip，limit)=>(4,4)，在这种情况下，mongo从所有文档中对文档进行排序，其中文档可以重复，而键值相同.

In second Aggregation you are again using (skip,limit) => (4,4) in this case mongo is sorting the documents from all of the document where documents can be duplicates while having same value in key.

因此，对数据进行排序后，应根据需要使用任何唯一键("_id"或名称")对数据进行排序 注意:金钥必须是唯一的

So after sorting by your your data you should sort your data with any unique key (either ‘_id’ or ‘name’) as you wish Note : key should be unique

类似下面的

db.testing.aggregate([
    {
        $sort : { 
          "percentage": -1,
          "_id" : 1
        },
    },
    {
        $skip : 0
    },
    {
        $limit:4
    }
]);

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