Support sorting by multiple columns

lib/src/postgrest_transform_builder.dart

@@ -45,7 +45,8 @@ class PostgrestTransformBuilder<T> extends PostgrestBuilder {
   PostgrestTransformBuilder order(String column,
       {bool ascending = false, bool nullsFirst = false, String? foreignTable}) {
     final key = foreignTable == null ? 'order' : '"$foreignTable".order';
-    final value =
+    final existingOrder = url.queryParameters[key];
+    final value = '${existingOrder == null ? '' : '$existingOrder,'}'
         '"$column".${ascending ? 'asc' : 'desc'}.${nullsFirst ? 'nullsfirst' : 'nullslast'}';
 
     appendSearchParams(key, value);

test/resource_embedding_test.dart

@@ -35,8 +35,21 @@ void main() {
         .execute();
     expect(res.data[0]['messages'].length, 2);
     expect(res.data[1]['messages'].length, 0);
-    expect(res.data[2]['messages'].length, 0);
-    expect(res.data[3]['messages'].length, 0);
+    expect(res.data[0]['messages'][0]['id'], 2);
+  });
+
+  test('embedded order on multiple columns', () async {
+    final res = await postgrest
+        .from('users')
+        .select('username, messages(*)')
+        .order('username', ascending: true)
+        .order('channel_id', foreignTable: 'messages')
+        .execute();
+    expect(res.data[0]['username'], 'awailas');
+    expect(res.data[3]['username'], 'supabot');
+    expect(res.data[0]['messages'].length, 0);
+    expect(res.data[3]['messages'].length, 2);
+    expect(res.data[3]['messages'][0]['id'], 2);
   });
 
   test('embedded limit', () async {

test/transforms_test.dart

@@ -15,6 +15,13 @@ void main() {
     expect(res.data[3]['username'], 'awailas');
   });
 
+  test('order on multiple columns', () async {
+    final res = await postgrest.from('users').select().order('catchphrase', ascending: true).order('username').execute();
+    expect(res.data[0]['username'], 'kiwicopple');
+    expect(res.data[2]['username'], 'supabot');
+    expect(res.data[3]['username'], 'dragarcia');
+  });
+
   test('limit', () async {
     final res = await postgrest.from('users').select().limit(1).execute();
     expect(res.data.length, 1);