anchor.diff:          ];
   
         // when
  -      handleDiff(diff, months.collection, 'author');
  +      handleDiff(diff, months, 'author');
         months.collection.addIndex(['_id']);
   
         // then
  
positive.code: import 'package:clean_data/clean_data.dart';
  import 'dart:async';
  
  class ConnectionMock extends Mock implements Connection {}
  class IdGeneratorMock extends Mock implements IdGenerator {}
  class CommunicatorMock extends Mock implements Communicator {}
  class FunctionMock extends Mock implements Function {}
  
negative.code:     });
  
      tearDown(() {
        if (months != null) months.dispose();
      });
  
      test("assign id to data.", () {
  