how can i agregate datas from different collections in mongodb using nodejs? - node.js

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 :)