Using Mongoose and Typescript, what type should be used for a ref field of an interface?

Question

I am using mongoose and Typescript, and I am wanting to know what type, or types, I should be using for a reference field, when creating an interface? Consider the following two related interfaces:

interface ICat {
  name: string,
  colour: string,
}

interface ICatDB extends ICat, Document {};

interface IMouse {
  name: string,
  colour: string,
  chasedBy: /* ... */
}

interface IMouseDB extends IMouse, Document {};

And the schemas and models that use them:

let cat = new Schema({
  name: String,
  colour: String,
});

mongoose.model<ICatDB>('Cat', cat);

let mouse = new Schema({
  name: String,
  colour: String,
  chasedBy: { type: Schema.Types.ObjectId, ref: 'Cat' }
});

mongoose.model<IMouseDB>('Mouse', mouse);

For the chasedBy field we need to consider that it can take values in three forms:

• String or ObjectId , when passed to a create() method
• ObjectId when returned from Mongoose
• Instance of ICat when returned from Mongoose, using populate()

Is there a way that we can specify the types the interface can support, without having to resort to using any ?

BTW we separated IMouse and IMouseDB , since Typescript wanted all the fields for Document filled out every time we created a new IMouse object, so this was a work around.

Answer 1

You could use this: https://mongoosejs.com/docs/typescript/populate.html

import { PopulatedDoc } from 'mongoose';
import ICat from './ICat';

interface IMouse {
  name: string,
  colour: string,
  chasedBy: PopulatedDoc<ICat>
}

Answer 2

Lacking any alternative answer, I went with:

import { ObjectID } from 'mongodb';
import ICat from './ICat';

interface IMouse {
  name: string,
  colour: string,
  chasedBy: string | ObjectID | ICat
}

This describes the type variants for the 'chasedBy' property. As to whether this is the recommended approach, I can't say.

Answer 3

Mongoose 6.x

Mongoose offers type declarations in mongoose.Types

interface Mouse { // important: don't use I in front of the name and don't extend Document
  chasedBy: mongoose.Types.ObjectId
}