Conditional selects

Sometimes you may want to select some fields based on a runtime condition. Something like this:

async function getPerson(id: number, withLastName: boolean) {}

If withLastName is true the person object is returned with a last_name property, otherwise without it.

Your first thought can be to simply do this:

async function getPerson(id: number, withLastName: boolean) {
  let query = db.selectFrom('person').select('first_name').where('id', '=', id)

  if (withLastName) {
    query = query.select('last_name')
  }

  // ❌ Wrong return type { first_name: string }
  return await query.executeTakeFirstOrThrow()
}

While that would compile, the result type would be { first_name: string } without the last_name column, which is wrong. What happens is that the type of query when created is something, let's say A. The type of the query with last_name selection is B which extends A but also contains information about the new selection. When you assign an object of type B to query inside the if statement, the type gets downcast to A.

You can write code like this to add conditional where, groupBy, orderBy etc. statements that don't change the type of the query builder, but it doesn't work with select, returning, innerJoin etc. that do change the type of the query builder.

In this simple case you could implement the method like this:

async function getPerson(id: number, withLastName: boolean) {
  const query = db
    .selectFrom('person')
    .select('first_name')
    .where('id', '=', id)

  if (withLastName) {
    // ✅ The return type is { first_name: string, last_name: string }
    return await query.select('last_name').executeTakeFirstOrThrow()
  }

  // ✅ The return type is { first_name: string }
  return await query.executeTakeFirstOrThrow()
}

This works fine when you have one single condition. As soon as you have two or more conditions the amount of code explodes if you want to keep things type-safe. You need to create a separate branch for every possible combination of selections or otherwise the types won't be correct.

This is where the $if method can help you:

async function getPerson(id: number, withLastName: boolean) {
  // ✅ The return type is { first_name: string, last_name?: string }
  return await db
    .selectFrom('person')
    .select('first_name')
    .$if(withLastName, (qb) => qb.select('last_name'))
    .where('id', '=', id)
    .executeTakeFirstOrThrow()
}

Any selections added inside the if callback will be added as optional fields to the output type since we can't know if the selections were actually made before running the code.