code: public BeanQuery<Map<String,Object>> nested(){
      if(!(this.selector instanceof KeyValueMapSelector)){
        throw new IllegalStateException("This is only for BeanQueries which has a KeyValueMapSelector selector. The current selector is a "+this.selector);
      }
      BeanQuery<Map<String,Object>> result= new BeanQuery<Map<String,Object>>(new NestedKeyValueMapSelector((KeyValueMapSelector)this.selector));
      result.from=this.from;
      result.predicate=this.predicate;
      result.comparator=this.comparator;
      result.descSorting=this.descSorting;
      return result;
    }
docs: Convert the select into a NestedKeyValueMapSelector.<br>
  Note<br>:
  This method is only available for BeanQueries with a KeyValuMapSelector. Calling this method on a BeanQeury with other types of selector will cause a IllegalStateExcewption.