Mongo聚合$filter在true/false上不起作用

Question

我有以下数据

{  
   "_id":"57b5271231a5a203f8815e78",
   "name":"Group A",
   "created":new Date(1471489810155),
   "active":true,
   "invites":[  
   ],
   "requests":[  
      {  
         "_id":"57b683e531a5a21679b78e6c",
         "created":new Date(1471579109215),
         "user_id":"57b4c5f0291ebb23110b888e",
         "description":"Request From John",
         "active":true,
         "date":         new Date("2016-08-23T13:58:15-0700         "), 
            "         denied_users":[  
         ]
      },
      {  
         "_id":"57b683e531a5a21679a78e6c",
         "created":new Date(1471579109215),
         "user_id":"57b4c5f0291ebb13110b888e",
         "description":"Request A",
         "active":true,
         "date":         new Date("2016-08-23T13:58:15-0700         "), 
            "         denied_users":[  
         ]
      },
      {  
         "_id":"57b6841231a5a21679a78e6d",
         "created":new Date(1471579154271),
         "user_id":"57b4c5f0291ebb13110b888e",
         "description":"Request B",
         "active":true,
         "date":         new Date("2016-08-26T13:59:07-0700         "), 
            "         denied_users":[  
         ]
      }
   ],
   "fulfillments":[  
   ]
}

对Retrieve all embedded Request objects that have active = true执行如下查询。

db.groups.aggregate(
  [
    { $project : { "requests" : 1 }},
    { $unwind : "$requests" },
    { $project: {
        "requests": {
           "$filter": {
              "input": "$requests",
              "as": "item",
              "cond": { $eq: [ "$$item.active",true ] }
          }
        }
      }
    }
  ]
)

我做错了什么?

如果我做下面的

是否会更容易(相同)?

[
     { $project : { "requests" : 1 }},
     { $unwind : "$requests" }, 
     { $match : { "requests.active" : true } }
]

Answer 1

您只需要从管道中删除$unwind级:

db.groups.aggregate(
  [
    { $project: {
        "requests": {
           "$filter": {
              "input": "$requests",
              "as": "item",
              "cond": { $eq: [ "$$item.active",true ] }
          }
        }
      }
    }
  ]
)

否则$unwind将requests字段从数组更改为单个对象，因为它复制了$filter不能使用的文档。