How to refine prisma type

Question

This is a very simplified case, I have more advanced cases. I have defined a "payment" model in my schema like this:

model Payment {
  id                      Int      @default(autoincrement()) @id
  covered                 Boolean?
  paid                    Boolean
  amount                  number
}

Doing const payments = Payment.findMany with all properties selected, this gives us a type of this:

type Payment = {
  id: number;
  covered?: boolean;
  paid: boolean;
  amount: number;
}

However I want to tell Prisma that findMany should actually return this below. In below, covered is non-null in case of paid being true :

type Payment = {
  id: number;
  amount: number;
} & (
  | { paid: false; covered?: never }
  | { paid: true; covered: boolean }
)

Is it possible to tell Prisma this refined type hint somehow? I tried this in the code below:

const payments = await prisma.payment.findMany(.....) as Omit<typeof payments[number], 'paid' | 'covered'> & (
  | { paid: false; covered?: never }
  | { paid: true; covered: boolean }
);

Answer 1

I think your best bet is to use what's called "discriminant type" using a common property, in your case paid property like this. Here's the link to the playground for your reference

interface BasePayment {
  id: number;
  paid: boolean;
  amount: number;
}

interface PaidPayment extends BasePayment {
  paid: true;
  covered: boolean;
}

interface UnpaidPayment extends BasePayment {
  paid: false;
}

const paidPayment: PaidPayment = {
  id: 1,
  amount: 10,
  covered: true,
  paid: true
};

const unpaidPayment: UnpaidPayment = {
  id: 1,
  amount: 10,
  covered: true, // This won't compile because type `UnpaidPayment` doesn't have `covered`
  paid: false
};