Mongoose Schema with Typescript - Design errors

Question

I have two problems defining schema using mongoose and typescript. Here is my code:

import { Document, Schema, Model, model} from "mongoose";

export interface IApplication {
    id: number;
    name: string;
    virtualProperty: string;
}

interface IApplicationModel extends Document, IApplication {} //Problem 1

let ApplicationSchema: Schema = new Schema({
    id: { type: Number, required: true, index: true, unique: true},
    name: { type: String, required: true, trim: true },
});
ApplicationSchema.virtual('virtualProperty').get(function () {
    return `${this.id}-${this.name}/`; // Problem 2
});
export const IApplication: Model<IApplicationModel> = model<IApplicationModel>("Application", ApplicationSchema);

First of all:

• Problem 1 in this line

interface IApplicationModel extends Document, IApplication {}

The Typescript is telling me that:

error TS2320: Interface 'IApplicationModel' cannot simultaneously extend types 'Document' and 'IApplication'. Named property 'id' of types 'Document' and 'IApplication' are not identical.

So how to change definition of id property?

• Problem 2 is in inner function (getter for virtualProperty ):

  return `${this.id}-${this.name}/; // Problem 2

The Error is:

error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.

How to define type of this ?

Answer 1

Problem #1: Since IApplicationModel extends interfaces Document and IApplication that declare the id property with different types ( any and number respectively), TypeScript does not know whether the id property of IApplicationModel should be of type any or number . You can fix this by redeclaring the id property in IApplicationModel with the desired type. (Why are you declaring a separate IApplication interface rather than just declaring IApplicationModel that extends Document with all your properties?)

Problem #2: Just declare the this special parameter to the function as shown below.

import { Document, Schema, Model, model} from "mongoose";

export interface IApplication {
    id: number;
    name: string;
    virtualProperty: string;
}

interface IApplicationModel extends Document, IApplication {
    id: number;
}

let ApplicationSchema: Schema = new Schema({
    id: { type: Number, required: true, index: true, unique: true},
    name: { type: String, required: true, trim: true },
});
ApplicationSchema.virtual('virtualProperty').get(function (this: IApplicationModel) {
    return `${this.id}-${this.name}/`;
});
export const IApplication: Model<IApplicationModel> = model<IApplicationModel>("Application", ApplicationSchema);

Answer 2

for problem 2: just declare this as any : .get(function (this: any) {}); fix it