Morphia实现类似select *,A from table的效果（A是虚拟字段）

经过我的一番研究：

public void test() {
   /*OOO类就是Model类*/
   Field[] fields = OOO.class.getDeclaredFields();
   List projections = new ArrayList<>();
   for(int i = 0; i < fields.length; i ++) {
       if(Modifier.isStatic(fields[i].getModifiers())) {/*去除静态变量*/
       continue;
       }
       projections.add(Projection.projection(fields[i].getName()));
   }
   /*虚拟字段A的值来源于X字段与O字段之和*/
   projections.add(Projection.projection("A", Projection.add("$X", "$O")));
   AggregationPipeline pip = miDao.createAggregation()
       .project(projections.toArray(new Projection[projections.size()]));"$peerNominateCount")))
       .sort(Sort.descending("A"));
   Iterator list = pip.aggregate(XXX.class);
   while(list.hasNext()) {
       System.out.println(list.next().toString());
   }
   }
/*XXX是专门为查询出的新数据设计的数据结构，当然你也可以把OOO类中的字段添加过来，或者直接继承它*/
private static class XXX{
   private static class TestModel{
   
   @Id
   private ObjectId id;
   
   private Long A;

   public ObjectId getId() {
       return id;
   }

   public void setId(ObjectId id) {
       this.id = id;
   }

   public Long getA() {
       return A;
   }

   public void setA(Long A) {
       this.A = A;
   }

   @Override
   public String toString() {
       return "TestModel [id=" + id + ", A=" + A + "]";
   }
   }

}