什么MongoDB的查询将限制嵌套列表

Question

我有以下文件的结果：

{ 
    "username":"test", 
    "comments": 
    [ 
     { 
      "commentId":"53453445", 
      "message":"Hello this is a message" 
     }, 
     { 
      "commentId":"53453445", 
      "message":"Hello this is a message" 
     }, 
     { 
      "commentId":"53453445", 
      "message":"Hello this is a message" 
     } 
    ] 
} 

如何限制的意见，只显示2项？

Answer 1

的$slice操作：

db.collection.find({ "username":"test" }, { comments: {$slice: 2 } });