feat: query parser: handle empty embedded resources ()

Closes #445.

src/select-query-parser.ts

@@ -202,6 +202,8 @@ type ConstructFieldDefinition<
       Field['children'],
       unknown
     >
+  : Field extends { children: [] }
+  ? {}
   : Field extends { name: string; original: string; hint: string; children: unknown[] }
   ? {
       [_ in Field['name']]: GetResultHelper<
@@ -460,19 +462,22 @@ type ParseJsonAccessor<Input extends string> = Input extends `->${infer Remainde
 
 /**
  * Parses an embedded resource, which is an opening `(`, followed by a sequence of
- * nodes, separated by `,`, then a closing `)`.
+ * 0 or more nodes separated by `,`, then a closing `)`.
  *
  * Returns a tuple of ["Parsed fields", "Remainder of text"], an error,
  * or the original string input indicating that no opening `(` was found.
  */
 type ParseEmbeddedResource<Input extends string> = Input extends `(${infer Remainder}`
   ? ParseNodes<EatWhitespace<Remainder>> extends [infer Fields, `${infer Remainder}`]
     ? EatWhitespace<Remainder> extends `)${infer Remainder}`
-      ? Fields extends []
-        ? ParserError<'Expected fields after `(`'>
-        : [Fields, EatWhitespace<Remainder>]
+      ? [Fields, EatWhitespace<Remainder>]
       : ParserError<`Expected ")"`>
-    : ParseNodes<EatWhitespace<Remainder>>
+    : // If no nodes were detected, check for `)` for empty embedded resources `()`.
+    ParseNodes<EatWhitespace<Remainder>> extends ParserError<string>
+    ? EatWhitespace<Remainder> extends `)${infer Remainder}`
+      ? [[], EatWhitespace<Remainder>]
+      : ParseNodes<EatWhitespace<Remainder>>
+    : ParserError<'Expected embedded resource fields or `)`'>
   : Input
 
 /**

test/index.test-d.ts

@@ -79,6 +79,15 @@ const postgrest = new PostgrestClient<Database>(REST_URL)
   )
 }
 
+// embedded resource with no fields
+{
+  const { data, error } = await postgrest.from('messages').select('message, users()').single()
+  if (error) {
+    throw new Error(error.message)
+  }
+  expectType<{ message: string | null }>(data)
+}
+
 // json accessor in select query
 {
   const { data, error } = await postgrest