fix modelName being used as value to partialFilterExpression index

[egorovli]

Summary

The issue is described here: #7634. If the third parameter is used in a Model.discriminator() method it should be used as a value to partialFilterExpression index instead of the model.modelName.

Examples

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
)

Before the fix db.payments.getIndexes() returns:

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

After the fix:

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

[egorovli]

I'm not really familiar with mongoose internals, maybe this && check isn't necessary.