使用$ unwind获取空数组作为聚合结果

问题描述

我有一个如下文档，使用猫鼬应用了聚合查询，该查询粘贴在json数据之后.在这里，我尝试使用子收集时间字段.查询结果为空数组，请帮助查找错误.查询可删除$unwind和相关的$project字段

I have a document like below, applied an aggregate query using mongoose which is pasted after json data. Here I am trying to use fields of sub collection- timing. Query results an empty array, please help to find mistake in query. Query works on removing $unwind and related $project field

{  
   "_id":"5b9cbeff67ea7bc00c9917a5",
   "route_index":0,
   "route_name":"NH85", 
   "access_type":"public",
   "type":"bus", 
   "duration_days":null,
   "duration_hours":0.23,
   "created_on":"2018-09-15T07:50:17.211Z",
   "title":"Rx100",
   "description":"Ls",
   "timings":[  
      {  
         "departure":"08:00",
         "destination":"08:23",
         "_id":"5b9cbeff67ea7bc00c9917a3",
         "trackings":[  

         ]
      },
      {  
         "departure":"10:00",
         "destination":"10:23",
         "_id":"5b9cbeff67ea7bc00c9917a2",
         "trackings":[  

         ]
      }
   ],
   "created_uid":"5b8f1a9db83bc9bf3d89dcc5", 
   "__v":0
}

我尝试过的查询在这里:

Query which I have tried is here:

Transport.aggregate([
        { $unwind: "$timings" } 
        {
            $project: {
                title: 1,  
                "trip_timings": "$timings"
            }
        }, 
        { $skip: ((perPage * page) - perPage) },
        { $limit: perPage }
    ], function (err, data) {
        if (err) {
            res.json(err);
        }
        res.json(data);
    });

推荐答案

尝试在$ unwind操作中将preserveNullAndEmptyArrays设置为true.可能是由于"trip_timings"字段包含空数组而导致结果被跳过. $ unwind的默认行为是忽略要取消展开的字段不存在或为空数组的文档.此处提供更多详细信息: https://docs.mongodb.com/manual /reference/operator/aggregation/unwind/#behaviors

Try setting preserveNullAndEmptyArrays to true in the $unwind operation. It might be that the results are being skipped because the "trip_timings" field contains empty arrays. The default behaviour of $unwind is to ignore documents where the field to be unwound does not exist or is an empty array. More details here: https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/#behaviors

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