Temporal types lose type information in query select projections

[goatrenterguy]

Bug Description

When a schema field is typed as Temporal.PlainDate (or any other Temporal type), using it in a .select() projection causes TypeScript to mangle the type — all methods collapse to {}.

Reproduction

import { Temporal } from 'temporal-polyfill'

type Todo = {
  id: number
  name: string
  dueDate: Temporal.PlainDate
  project_id: number
}

type Project = {
  id: number
  name: string
}

const todos = createCollection<Todo>(/* ... */)
const projects = createCollection<Project>(/* ... */)

const liveQuery = createLiveQueryCollection({
  query: (q) =>
    q
      .from({ todo: todos })
      .innerJoin({ project: projects }, ({ todo, project }) =>
        eq(todo.project_id, project.id),
      )
      .select(({ todo, project }) => ({
        todo,
        project,
      })),
})

// Type error: dueDate is mangled
const date: Temporal.PlainDate = liveQuery.toArray[0]!.todo.dueDate

Error:

Type '{ readonly [Symbol.toStringTag]: "Temporal.PlainDate"; toString: {}; equals: {};
  add: {}; subtract: {}; ... }' is not assignable to type 'PlainDate'.
  Types of property 'equals' are incompatible.
    Type '{}' is not assignable to type '(other: string | PlainDate | PlainDateLike) => boolean'.

Root Cause

In packages/db/src/query/builder/types.ts, IsPlainObject<T> determines how each field is processed through the Ref<T> mapped type:

• Plain objects → recursively walked to build nested refs
• Non-plain objects (arrays, JsBuiltIns like Date, Map, etc.) → treated as leaf values, type preserved as-is

Temporal.PlainDate extends object but is not in the JsBuiltIns union, so IsPlainObject returns true. The type system then recursively walks its properties through Ref<T>, which turns all method signatures into {}.

This affects all Temporal types: PlainDate, PlainTime, PlainDateTime, ZonedDateTime, Instant, Duration, etc.

Suggested Fix

Add a Symbol.toStringTag check to IsPlainObject:

type IsPlainObject<T> = T extends unknown
  ? T extends object
    ? T extends ReadonlyArray<any>
      ? false
      : T extends JsBuiltIns
        ? false
        : T extends { [Symbol.toStringTag]: string }
          ? false
          : true
    : false
  : false

Objects with Symbol.toStringTag are class instances (Temporal types, typed arrays, etc.), not plain data objects. This covers all Temporal types without requiring imports or maintaining a hardcoded list.