code: public ModelDiff diff() {
          if (diff != null) return diff;
  
          setOffline();
  
          setDefaults();
  
          try {
              migrationModel = new MigrationModel(server);
              ModelContainer migrated = migrationModel.read();
  
              currentModel = new CurrentModel(server, constraintNaming);
              ModelContainer current = currentModel.read();
              diff = new ModelDiff(migrated);
              diff.compareTo(current);
          } finally {
              if (!online) {
                  DbOffline.reset();
              }
          }
          return diff;
      }
docs: <p>diff.</p>
  
  @return a {@link io.ebeaninternal.dbmigration.model.ModelDiff} object.