Implemented whereRelatedTo query (parse-community#192)

* Implemented querying for related objects defined based on a field of type Relation

* Implemented unit test for relatedTo query

Author: sidiabale

lib/src/network/parse_query.dart

@@ -138,6 +138,12 @@ class QueryBuilder<T extends ParseObject> {
         MapEntry<String, dynamic>(column, value), '\$exists'));
   }
 
+  /// Retrieves related objets where [String] column is a relation field to the class [String] className
+  void whereRelatedTo(String column, String className, String objectId) {
+    queries.add(MapEntry<String, dynamic>(_SINGLE_QUERY,
+        '\"\$relatedTo\":{\"object\":{\"__type\":\"Pointer\",\"className\":\"$className\",\"objectId\":\"$objectId\"},\"key\":\"$column\"}'));
+  }
+
   /// Returns an object where the [String] column contains select
   void selectKeys(String column, dynamic value) {
     queries.add(_buildQueryWithColumnValueAndOperator(

pubspec.yaml

@@ -29,5 +29,6 @@ dependencies:
 dev_dependencies:
   # Testing
   test: ^1.5.1
+  mockito: ^4.1.0
   flutter_test:
     sdk: flutter

test/parse_query_test.dart

@@ -0,0 +1,31 @@
+import 'package:mockito/mockito.dart';
+import 'package:parse_server_sdk/parse_server_sdk.dart';
+import 'package:test/test.dart';
+
+class MockClient extends Mock implements ParseHTTPClient {}
+
+void main() {
+  group("queryBuilder", () {
+    test('whereRelatedTo', () async {
+      final client = MockClient();
+
+      Parse().initialize('appId', 'https://test.parse.com', debug: true);
+
+      var queryBuilder =
+          QueryBuilder<ParseObject>(ParseObject('_User', client: client));
+      queryBuilder.whereRelatedTo('likes', 'Post', '8TOXdXf3tz');
+
+      when(client.data).thenReturn(ParseCoreData());
+      await queryBuilder.query();
+
+      Uri result = (verify(client.get(captureAny)).captured.single as Uri);
+
+      expect(result.path, "/classes/_User");
+
+      Uri expectedQuery = Uri(
+          query:
+              'where={"\$relatedTo":{"object":{"__type":"Pointer","className":"Post","objectId":"8TOXdXf3tz"},"key":"likes"}}');
+      expect(result.query, expectedQuery.query);
+    });
+  });
+}