如何使用nodejs从mongodb中的不同collections聚合数据？

Question

I am using mongoDB and nodejs I have 4 collections collection 1 is teacher with fields teacher_id, teacher_name collection 2 is subject with fieldssubject _id, subject_name collection 3 is book with fields book_id, book_name

collection is student which have fields -- _id, student_name, teacher_id, subject_id, book_id

how can I fetch ids from 1, 2, 3 collections simultaneously and insert to corresponding id in collection

I have tried some which always ask for a matching field... is ther any function which returns data from collection even though no match field?

can someone please help

Answer 1

Well, in that case, you need to fetch all the documents from those collections. That will be a bit costly aggregation but I'm adding here in code:

Firstly, I'm grouping on null, to avoid to attach lookup value to every single document in teacher collection.

db.teacher.aggregate([
  {
    $group:{
      "_id":null,
      "root":{
        $push:"$$ROOT"
      }
    }
  },
  {
  $lookup:
   {
     from:"subject",
     pipeline: [],
     as: "subjectLookup"
   }
  },
  {
  $lookup:
   {
     from:"book",
     pipeline: [],
     as: "bookLookup"
   }
  },
  {
  $lookup:
   {
     from:"student",
     pipeline: [],
     as: "studentLookup"
   }
  }
]).pretty()

These lookups will give the array which contains all the documents from respective collections, you can limit the documents by adding $match stage in the pipeline of lookup stage.

Hope this will solve your problem:)