Mongoose: Can't use same discriminator in multiple models

[joniba]

Do you want to request a feature or report a bug?
To me it seems like a bug, but probably it will end up being a feature request.

What is the current behavior?
Trying to define the same discriminator for two different models on two different collections throws an OverwriteModelError: Cannot overwrite[model name]model once compiled.

If the current behavior is a bug, please provide the steps to reproduce.`

const mongoose = require('mongoose');
const { Schema } = mongoose;
const DB = '7100';
const URI = `mongodb://localhost:27017/${DB}`;
const OPTS = { family: 4, useNewUrlParser: true };

const DocumentSchema = new Schema({
});

const InvoiceSchema = new Schema({
});

const DeliveryNoteSchema = new Schema({
});


const FactSchema = new Schema({
});

const InvoiceFactSchema = new Schema({
});

const DeliveryNoteFactSchema = new Schema({
});

const Document = mongoose.model('Document', DocumentSchema);
const Invoice = Document.discriminator('Invoice', InvoiceSchema);
const DeliveryNote = Document.discriminator('DeliveryNote', DeliveryNoteSchema);

const Fact = mongoose.model('Fact', FactSchema);
const InvoiceFact = Fact.discriminator('Invoice', InvoiceFactSchema);
const DeliveryNoteFact = Fact.discriminator('DeliveryNote', DeliveryNoteFactSchema);

mongoose.connect(URI, OPTS);

What is the expected behavior?
No error

Please mention your node.js, mongoose and MongoDB version.
node v8.10.0
mongoose 5.3.2
MongoDB 3.2

[mhombach]

You have 2 typos which are causing this error.
The 2 Models which are discriminated from Fact need to be named like this:

const Fact = mongoose.model('Fact', FactSchema);
const InvoiceFact = Fact.discriminator('InvoiceFact', InvoiceFactSchema);
const DeliveryNoteFact = Fact.discriminator('DeliveryNoteFact', DeliveryNoteFactSchema);

I call it "typo" because you named them correct on the Document-Discriminators above.
With this minimal correction the error disappears :)

[joniba]

It's not a typo. It's the way the system was built and cannot easily be changed without modifying both servers and a variety of clients, not to mention millions of records of historical data. I don't understand why mongoose would prevent me from using the same discriminator in two different models. They are in separate collections so there should be no conflict.

[mhombach]

Ok, or you misread the documentation.
You are not specifying and discriminator-key in your code.
The first parameter in the discriminator-function is the name of the model.
So this line here

Fact.discriminator('Invoice', InvoiceFactSchema)

tries to create a model with the name "Invoice", regardless of how you name the variable.
But a a model (not "variable", i am talking about the mongoose-internally model-name) with the name "Invoice" does already exist because of this line

Document.discriminator('Invoice', InvoiceSchema)

You can still give a 3rd parameter to the discriminator-function which then speficies the value of the discriminator-key.
Maybe this link here clarifies things: https://mongoosejs.com/docs/api.html#model_Model.discriminator
If you have further questions, feel free to ask :)

[joniba]

Oh, I understand now, thank you!

[vkarpov15]

Related issues: https://github.com/Automattic/mongoose/issues/4148 , https://github.com/Automattic/mongoose/issues/2596, https://github.com/Automattic/mongoose/commit/10efec326a0cbcc04054ed7f56cb4886de1009cd .

Worth thinking about more for the next backwards breaking release. We have a few options for this:

1) Make discriminator names dotted, so Document.discriminator('Invoice') would compile a model with name Document.Invoice
2) Don't put discriminators in top-level models() at all. To access a discriminator, you'd instead have to get the model and then the discriminator, something like mongoose.model('Document').discriminator('Invoice').find().

I'm not thrilled about either of these solutions honestly. Anyone have any strong thoughts on this - @joniba ? It looks like the value argument to Model.discriminator() should solve your problem.

[gfpacheco]

Also the main page for discriminators in the doc says This function takes 2 parameters when it takes 3 actually: https://mongoosejs.com/docs/discriminators.html#the-model-discriminator-function

[giorni]

I had a similar problem, which is solved by using the 3rd parameter, but it was hard to debug because no error was thrown due the way I defined and used my models. The create call hang without doing anything. I am posting here just as reference for anyone that may have created the environment the same way as I did.

const mongoose = require('mongoose')
mongoose.connect("...")
const { Schema } = mongoose;

const S1 = new Schema({}, {discriminatorKey: 'kind' })
const S1_1 = new Schema({})
const S1_2 = new Schema({})

const M1 = mongoose.model('m1', S1)
M1.discriminator('M2', S1_1)
M1.discriminator('M3', S1_2)

// This is necessary to avoid model compilation errors in watch mode
// see https://github.com/Automattic/mongoose/issues/1251
if (mongoose.modelNames().includes('M3')) {
  mongoose.deleteModel('M3')
}

const S2 = new Schema({}, {discriminatorKey: 'kind'})
const M3 = mongoose.model('M3', S2)

M1.create([{kind: 'M2}]) // Promise hangs, no error thrown.