MongoDB: 如何匹配数组的元素？

Question

我有两个合集如下：

db.qnames.find()
{ "_id" : ObjectId("5a4da53f97a9ca769a15d49e"), "domain" : "mail.google.com", "tldOne" : "google.com", "clients" : 10, "date" : "2016-12-30" }
{ "_id" : ObjectId("5a4da55497a9ca769a15d49f"), "domain" : "mail.google.com", "tldOne" : "google.com", "clients" : 9, "date" : "2017-01-30” }

和

db.dropped.find()
{ "_id" : ObjectId("5a4da4ac97a9ca769a15d49c"), "domain" : "google.com", "dropDate" : "2017-01-01", "regStatus" : 1 }

我想加入两个集合并选择 'dropDate' 字段（来自丢弃的集合）大于 'date' 字段（来自 qnames 字段）的文档。所以我使用了以下查询：

 db.dropped.aggregate( [{$lookup:{ from:"qnames", localField:"domain",foreignField:"tldOne",as:"droppedTraffic"}},
     {$match: {"droppedTraffic":{$ne:[]}  }}, 
{$unwind: "$droppedTraffic" } , 
{$match: {dropDate:{$gt:"$droppedTraffic.date"}}} ])

但此查询不会过滤 dropDate < date 的记录。谁能告诉我为什么会这样？

Answer 1

您应该使用 $redact 来比较同一文档的两个字段。以下示例应该有效：

db.dropped.aggregate( [
    {$lookup:{ from:"qnames", localField:"domain",foreignField:"tldOne",as:"droppedTraffic"}},
    {$match: {"droppedTraffic":{$ne:[]}  }}, 
    {$unwind: "$droppedTraffic" }, 
    {
        "$redact": {
            "$cond": [
                { "$lte": [ "$dropDate", "$droppedTraffic.date" ] },
                "$$KEEP",
                "$$PRUNE"
            ]
        }
    }
])

Answer 2

您收不到记录的原因是

Date is used as a String in your collections, to make use of the comparison operators to get the desired result modify your collection documents using new ISODate("your existing date in the collection")

请注意，即使在修改了两个集合之后，您也需要修改聚合查询，因为在最终的 $match 查询中，比较了同一文档中的两个值。

获取所需文档的示例查询

db.dropped.aggregate([ 
    {$lookup: { 
       from:"qnames", 
       localField:"domain", 
       foreignField:"tldOne", 
       as:"droppedTraffic"}
    },
    {$project: {
       _id:1, domain:1, 
       regStatus:1,
       droppedTraffic: {
          $filter: { 
             input: "$droppedTraffic", 
             as:"droppedTraffic", 
             cond:{ $gt: ["$$droppedTraffic.date", "$dropDate"]}
          }
       }
     }} 
])

在上面给出的这种方法中，我们使用了$filter which avoids the $unwind操作

MongoDB: 如何匹配数组的元素？

MongoDB: How to match on the elements of an array?

lookup

match

mongodb