How to type a generic selection function

I've been trying to implement a generic select function but can't figure out how to type it Something like this:

async function select<T extends Something>(selectFields: T, limit: number): SomethingReturn> {
  const db = await getClient()
  const res = await db.select(selectFields).from(users).limit(limit)
  return res
}

const users = await select({ name: users.name }, 10)
// Should be typed `{ name: string }[]`

async function select<T extends Something>(selectFields: T, limit: number): SomethingReturn> {
  const db = await getClient()
  const res = await db.select(selectFields).from(users).limit(limit)
  return res
}

const users = await select({ name: users.name }, 10)
// Should be typed `{ name: string }[]`

What type should I use to replace Something and SomethingReturn I tried a combination of

type TSchema = ExtractTablesWithRelations<typeof schema>
type TFields = TSchema['users']['columns']

type TSchema = ExtractTablesWithRelations<typeof schema>
type TFields = TSchema['users']['columns']

and multiple combinations of PgSelect , PgSelectBase but can't get it to work properly Any help appreciated

11 Replies

Mario564•this hour

@alexb Use the SelectedFields type (T extends SelectedFields)

alexb•this hour

@Mario564 thanks SelectedFields really help but I'm still having issues It works unless I add a where condition ie: return db.select(selectFields).from(users) // works return db.select(selectFields).from(users).where(eq(users.id, '1111')) // returns "any"

Mario564•this hour

Where is the SelectedFields being imported from? I'm looking at the source code and had forgotten there's one for each dialect, so you have to use the correct one for whatever dialect you're using

alexb•this hour

from import type { SelectedFields } from 'drizzle-orm/pg-core'

Mario564•this hour

That should work Are you still explicitly defining the return type for the function?

alexb•this hour

Nope, letting typescript infer it Attaching a more concrete example

Mario564•23h ago

Couldn't find a clean solution to this problem, but I did come up with a workaround

async function select<T extends SelectedFields>(fields: T) {
  const query = db.select(fields).from(users);
  return await query.where(eq(users.id, 1)) as Awaited<typeof query>;
}

async function select<T extends SelectedFields>(fields: T) {
  const query = db.select(fields).from(users);
  return await query.where(eq(users.id, 1)) as Awaited<typeof query>;
}

alexb•23h ago

Nice thanks! yeah this is exactly the workaround I used

async function select<T extends SelectedFields>(selectFields: T) {
  const drizzle = await getClient()
  const query = drizzle.select(selectFields).from(users)
  const queryWithWhere = query.where(eq(users.id, '123456')) as unknown as typeof query
  const res = await queryWithWhere
  return res
}

async function select<T extends SelectedFields>(selectFields: T) {
  const drizzle = await getClient()
  const query = drizzle.select(selectFields).from(users)
  const queryWithWhere = query.where(eq(users.id, '123456')) as unknown as typeof query
  const res = await queryWithWhere
  return res
}

That'll do, I feel like this is a pretty standard use case though, surprised there's no clean way to do it :/

Mario564•23h ago

There probably is but it's not documented anywhere at the moment The select types are also very complex so that certainly doesn't help

alexb•23h ago

Thanks for your help! I'll update the thread if I ever find a better solution

Mario564•23h ago

Alright, glad to have helped!

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How to type a generic selection function