Property does not exist on populated document mongoose - node.js

Question

Let's say I have two schemas:
Foo.ts
import mongoose, { Schema, Document } from 'mongoose';
export interface IFoo extends Document {
name: string;
}
const fooSchema = new Schema(
{
name: {
type: String,
required: true,
}
}
);
export default mongoose.model<IFoo>('Foo', fooSchema);
And Bar.ts
import mongoose, { Schema, Document } from 'mongoose';
export interface IBar extends Document {
fooId: string | IFoo; // can be string or can be Foo document
}
const barSchema = new Schema(
{
fooId: {
type: Schema.Types.ObjectId,
ref: 'Foo',
required: true,
},
title: String;
}
);
export default mongoose.model<IBar>('Bar', barSchema);
Now when I find a Bar document with Foo populated. I get a compilation error from typescript
const bar = await Bar.findOne({ title: 'hello' }).populate({ path: 'fooId', model: 'Foo' });
bar.fooId.name // here typescript gives an error
The error is
Property 'name' does not exist on type 'string'
Since I've defined in IBar that fooId can be string | IFoo. Why typescript is complaining? And how to solve it?

Answer 1

If you check typings of the populate method you can see that it's result type is simply this. So, before addressing name field of the fooId property you have to explicitly narrow the type of the result to IFoo type:
const bar = await Bar.findOne({ title: 'hello' })
.populate({ path: 'fooId', model: 'Foo' });
if (typeof bar.fooId !== 'string') { // discards the `string` type
bar.fooId.name
}
Or just simply type assert the result:
const bar = await Bar.findOne({ title: 'hello' })
.populate({ path: 'fooId', model: 'Foo' });
(bar.fooId as IFoo).name