MongoDB ObjectId 与 String 的关联

场景：

现有两个集合如下：

company: {_id: ObjectId("..."), countryId: "566eb3c704c7b31facbb0007"}
country: { _id: ObjectId("566eb3c704c7b31facbb0007"), name:"..."} 

正常关联：

{ $lookup: {
       from: "country",
       localField: "countryId",    <= string
       foreignField: "_id",    <= ObjectId
       as: "country"
}}

但结果却是无效

解决方案：

1. 使用 $project

   db.company.aggregate([
     {
       "$project": {
         "countryId": {
           "$toObjectId": "$countryId"
         }
       }
     },
     {
       "$lookup": {
         "from": "country",
         "localField": "countryId",
         "foreignField": "_id",
         "as": "country"
       }
     }
   ])
   

   或：

   db.company.aggregate([
     {
       "$project": {
         "_id": {
           "$toString": "$_id"
         }
       }
     },
     {
       "$lookup": {
         "from": "country",
         "localField": "countryId",
         "foreignField": "_id",
         "as": "country"
       }
     }
   ])
   

2. 使用 $lookup 的 let 属性

   db.company.aggregate([
     { 
         "$lookup": {
             "let": { "countryId": { "$toObjectId": "$countryId" } },
             "from": "country",
             "pipeline": [
               { "$match": { "$expr": { "$eq": [ "$_id", "?countryId" ] } } }
              ],
             "as": "country"
       }
     }
   ])
   

3. 使用 $addFields

   db.role.aggregate([
     { "$addFields": { "countryId": { "$toObjectId": "$countryId" }}},
     { "$lookup": {
       "from": "country",
       "localField": "countryId",
       "foreignField": "_id",
       "as": "country"
     }}
   ])