wrong partialFilterExpression index with discriminators

[egorovli]

Do you want to request a feature or report a bug?

A bug.

What is the current behavior?

The docs say it is possible to use three parameters to Model.discriminator(). If the third one is specified, it is used as a value for the discriminator. However the generated index in MongoDB uses the first parameter (the model name).

Consider the following example:

let BasePaymentSchema = new mongoose.Schema({
  amount: mongoose.Types.Decimal128
}, {
  discriminatorKey: 'provider'
})

let BasePayment = mongoose.model('Payment', BasePaymentSchema)

let PaypalPaymentSchema = new mongoose.Schema({
  transactionId: {
    type: String,
    unique: true
  }
})

let PaypalPayment = BasePayment.discriminator(
  'PaypalPayment',
  PaypalPaymentSchema,
  'paypal' // This value should be in discriminator field
)

But if I do db.payments.getIndexes() I get:

{
  "v": 2,
  "unique": true,
  "key": {
    "transactionId": 1
  },
  "name": "transactionId_1",
  "ns": "my-database.payments",
  "background": true,
  "partialFilterExpression": {
    "provider": "PaypalPayment"
  }
}

So basically the model name (PaypalPayment) is used as a partialFilterExpression instead of the supplied discriminator value (paypal). This way the unique constraint doesn't work at all.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node 11.12.0
Mongoose 5.4.19
MongoDB 4.0.6