@Override
 public ArrayList<Relation> queryRelation(
     Class class1, Class class2, List<String> key1List, List<String> key2List) {
   acquireReference();
   try {
     SQLStatement stmt = SQLBuilder.buildQueryRelationSql(class1, class2, key1List, key2List);
     final EntityTable table1 = TableManager.getTable(class1);
     final EntityTable table2 = TableManager.getTable(class2);
     final ArrayList<Relation> list = new ArrayList<Relation>();
     Querier.doQuery(
         mHelper.getReadableDatabase(),
         stmt,
         new Querier.CursorParser() {
           @Override
           public void parseEachCursor(SQLiteDatabase db, Cursor c) throws Exception {
             Relation relation = new Relation();
             relation.key1 = c.getString(c.getColumnIndex(table1.name));
             relation.key2 = c.getString(c.getColumnIndex(table2.name));
             list.add(relation);
           }
         });
     return list;
   } finally {
     releaseReference();
   }
 }