fix addModel and add test, expose current model (#110)

Follow up to #105 (comment)

pkgs/dart_model/lib/src/scopes.dart

@@ -159,12 +159,17 @@ class MacroScope {
   /// multiple queries.
   void addModel(Model model) {
     if (_accumulatedModel case var accumulated?) {
-      accumulated.mergeWith(model);
+      _accumulatedModel = accumulated.mergeWith(model);
     } else {
       _accumulatedModel = model;
     }
   }
 
+  /// The current accumulated model for this macro scope.
+  ///
+  /// This is only safe to use after [addModel] has been called at least once.
+  Model get model => _accumulatedModel!;
+
   static MacroScope get current {
     final scope = Scope._currentOrNull;
     return switch (scope) {

pkgs/dart_model/test/scopes_test.dart

@@ -7,7 +7,7 @@ import 'package:dart_model/src/json_buffer/json_buffer_builder.dart';
 import 'package:test/test.dart';
 
 void main() {
-  group(Scope, () {
+  group('Scope', () {
     for (final scope in [Scope.none, Scope.macro, Scope.query]) {
       test('create maps and serialize work in $scope', () {
         expect(scope.run(() => Scope.createMap(TypedMapSchema({}))),
@@ -33,5 +33,23 @@ void main() {
       Scope.macro.run(() => Scope.none.run(() {}));
       Scope.query.run(() => Scope.none.run(() {}));
     });
+
+    test('addModel merges the model with the previous', () {
+      late Model initial;
+      late Model firstQuery;
+      Scope.query.run(() {
+        initial = Model()..uris['a'] = (Library()..scopes['A'] = Interface());
+        firstQuery = Model()
+          ..uris['a'] = (Library()..scopes['B'] = Interface());
+      });
+      Scope.macro.run(() {
+        MacroScope.current.addModel(initial);
+        expect(MacroScope.current.model.uris['a']!.scopes['A'], isNot(null));
+        expect(MacroScope.current.model.uris['a']!.scopes['B'], null);
+        MacroScope.current.addModel(firstQuery);
+        expect(MacroScope.current.model.uris['a']!.scopes['A'], isNot(null));
+        expect(MacroScope.current.model.uris['a']!.scopes['B'], isNot(null));
+      });
+    });
   });
 }