$ Moongoose match amount does not match id

Question

I want to show products using identifiers ( 56e641d4864e5b780bb992c6 and 56e65504a323ee0812e511f2 ) and show the price after subtraction with a discount, if available.

I can calculate the final price using an aggregate, but this returns the whole document in the collection, how to make it return only matches ids

 "_id" : ObjectId("56e641d4864e5b780bb992c6"), "title" : "Keyboard", "discount" : NumberInt(10), "price" : NumberInt(1000) "_id" : ObjectId("56e65504a323ee0812e511f2"), "title" : "Mouse", "discount" : NumberInt(0), "price" : NumberInt(1000) "_id" : ObjectId("56d90714a48d2eb40cc601a5"), "title" : "Speaker", "discount" : NumberInt(10), "price" : NumberInt(1000) 

this is my request

 productModel.aggregate([ { $project: { title : 1, price: { $cond: { if: {$gt: ["$discount", 0]}, then: {$subtract: ["$price", {$divide: [{$multiply: ["$price", "$discount"]}, 100]}]}, else: "$price" } } } } ], function(err, docs){ if (err){ console.log(err) }else{ console.log(docs) } }) 

and if I add this request to $in , it returns an empty array

 productModel.aggregate([ { $match: {_id: {$in: ids}} }, { $project: { title : 1, price: { $cond: { if: {$gt: ["$discount", 0]}, then: {$subtract: ["$price", {$divide: [{$multiply: ["$price", "$discount"]}, 100]}]}, else: "$price" } } } } ], function(err, docs){ if (err){ console.log(err) }else{ console.log(docs) } }) 

Answer 1

Your ids variable will be built from string values, not ObjectId .

Mongoose "autocasts" string values ​​for ObjectId in their correct type in regular queries, but this does not happen in the aggregation pipeline , as described in issue number 1399.

Instead, you should do the right casting to manually enter:

 ids = ids.map(function(el) { return mongoose.Types.ObjectId(el) }) 

Then you can use them in your pipeline:

 { "$match": { "_id": { "$in": ids } } } 

The reason is that aggregation pipelines “usually” change the structure of the document, and therefore mongoose makes no assumptions that the “schema” applies to the document at any stage of the pipeline.

It can be argued that the stage of the "first" pipeline, when it is the $match stage, should do this, since the document does not really change. But now this is not so.

Any values ​​that may be "strings" or at least not valid BSON types must be manually added to match.