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

我有MongoDB的查询

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

如果申请者的ID为空，它将不起作用。这不是我想要的，如何使$lookup成为可选的？

最佳答案

$lookup执行外部联接，如果没有匹配，则不会进行筛选。
问题在于$unwind，它将在数组为空或为空时进行筛选，为了不筛选需要在preserveNullAndEmptyArrays : true管道中指定$unwind的空数组

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