Help with typing for accepting optional select columns via function arguments

[DanielBaulig]

I've been trying to wrap my head around this for over a day now. I searched, read documentation for Typescript and Drizzle up and down, tried using ChatGPT, experimented with various solutions. Nothing was able to give me a solution. I am trying to write a loadUser function that loads a user from the db. As an optional argument I want to provide the function a set of columns to include in the resulting row in addition to the default specified in the actual function. Here's what I currently have which appears to be working:

type UserCoreColumnKeys = 'id' | 'email' | 'name';
type UserCoreColumns = Pick<typeof users, UserCoreColumnKeys>;
type UserColumns = typeof users._.columns[keyof typeof users._.columns];
export async function loadUser<
  F extends {[key in K]: C},
  K extends Exclude<string, UserCoreColumnKeys>,
  C extends UserColumns,
>(id: string | SQL, fields: F) {
  const where = typeof id === 'string' ? eq(users.id, id) : id;

  const [user] = await db.select({
    ...fields,
    id: users.id,
    email: users.email,
    name: users.name,
  }).from(users).where(where).limit(1);

  return user;

I can call this like follows and it appears to work:

const user = await loadUser('userid', { someColumn: users.someColumn });
// user.someColumn and other core columns exist with right type.

As soon as I make the fields argument optional though, everything falls apart and the typing on the return user object is missing the properties specified on the fields object. I'm not sure why or how to prevent that. I've also tried a couple approaches of using conditional types and explicit return types, but I couldn't get the explicit return types to work at all. I tried using SelectResult<F & UserCoreColumns> and various conditional variations thereof, but the value actually returned from db.select never matched my attempts to explicitly define the return type. The types are so large and complex that I find it really hard to parse the error messages in a meaningful way. I am suspecting that the type mismatch when explicitly trying to define the return type might be due to how typescript merges object types, but I'm not sure.

I'd really love so help getting this to work since I'm at the end of my rope -- and nerves -- here.