MongoDB query using different collections?

Question

I saw other examples but still can't get how to do a query using multiple collections in MongoDB. For example, I have two collections, student and wand. Each student has a wand, and each wand has a code. Suppose I want to print all the wands of all the students who are from the house 'Slytherin'. How should I do it?

Answer 1

when create the model, use the student _id as a foreign key in the wand model like this

    let wand_schema = new Schema({ student_id : ObjectId, code : String, name : String});
module.exports = mongoose.model('wands', wand_schema);

the query.

let filter = { $match : {house : slytherin } }
let lookup = { $lookup : {
  from : "wands",
  localField : _id,
  foreignField : student_id,
  as : "wands"
}}
let student_wands = await student_model.aggregate([ filter, lookup ]);

the results will contain a field called wands which is an array of wands from the wands collection