Association between the MongoDB ObjectId and a String

Scene:

There are two collections as follows:

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

Normal association:

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

But it turned out to be ineffective

Solution:

1. Using the $project

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

   Or:

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

2. Use the let property of $lookup

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

3. Using the $addFields

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