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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?

Tomoko
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  • You probably want to look at aggregation pipelines and the `$lookup` operator you can use in there: https://stackoverflow.com/a/33511166/14955 – Thilo Jul 10 '20 at 09:07
  • I tried this db.wand.aggregate([ {$match: {house: 'slytherin'}}, { $lookup:{ from:"student", localField:"wandID", foreignField:"wandID", as:"slytherin1" } }); – Tomoko Jul 10 '20 at 09:35
  • but it doesnt work – Tomoko Jul 10 '20 at 09:35

1 Answers1

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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

warren chilambe
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