discriminated-union.ts

type ModelTypes = 'product' | 'order';

type Product = {productName: string};
type Order = {orderId: number};

type Model = {
    id: number;
    type: ModelTypes;
    entity: Product | Order;
}

const model: Model = {
    id: 1,
    type: 'order',
    entity: {
        // see, a mistake, the type is `order`,
      	// but we actually have a product here
        productName: 'Product'
    } 
}

if (model.type === 'product') {
    // this will give us an error as the type is `Product | Order`
    console.log(model.entity.productName);
}

// instead, use discriminated unions:

type Model = {
    id: number;
} & ({type: 'order', entity: Order} | {type: 'product', entity: Product});

const model: Model = {
    id: 1,
    type: 'order',
    entity: {
        // now, we'll get an error here, 
      	// as `productName` is incompatible with type `order`
        productName: 'Product'
    } 
}

if (model.type === 'product') {
    // no error here, type `product` ensure the `entity` field is a `Product`
    console.log(model.entity.productName);
}