node.js - mongodb/mongoose 使聚合 $lookup 成为可选的

Question

我有这个mongodb查询

Job.aggregate([ 
       {$match: {created_by: req.params.userId }},
       {$lookup: {
       from: "users", 
       localField: "applicant._id", 
       foreignField: "_id", 
       as: "applicant" }},
       {$unwind: '$applicant'}
     ])

applicant._id 为空时无效。这不是我想要的，如何使 $lookup 成为可选的？

Answer 1

$lookup 做外连接，不匹配则不过滤。

问题出在 $unwind 上，它会在数组为 null 或空时进行过滤，为了不过滤空数组，您需要指定 preserveNullAndEmptyArrays : true在 $unwind 管道中

Job.aggregate([ 
       {$match: {created_by: req.params.userId }},
       {$lookup: {
           from: "users", 
           localField: "applicant._id", 
           foreignField: "_id", 
           as: "applicant" }
       },
       {$unwind: {
           path :'$applicant', 
           preserveNullAndEmptyArrays: true}
       }
     ])