如何在隐藏某些字段的数组中发布某些元素？

Question

我有这个测验文件：

{
  _id: 001,
  quiz: "cartoon familes",
  questions:[
    {
      _id: 100,
      question: "who is the eldest ?",
      order: 1,
      answers: [
                  { _id: 101, answer: "Bart Simpson", points: 100 },
                  { _id: 102, answer: "Lisa Simpson", points: 0 },
                  { _id: 103, answer: "Maggie Simpson", points: 0 }
                ]
    },
    {
      _id: 200,
      question: "who is the eldest ?",
      order: 2,
      answers: [
                  { _id: 201, answer: "Chris Griffin", points: 0 },
                  { _id: 202, answer: "Meg Griffin", points: 100 },
                  { _id: 203, answer: "Stewie Griffin", points: 0 }
                ]
     }
  ]
}

我只想得到具有order: 1 字段的问题（所有其他问题都将被隐藏），我还希望points 字段被隐藏。

我该怎么做？

Answer 1

您可以在这样的聚合查询中使用两个$project 阶段来实现这一点：

db.collection.aggregate([  
   {  
      $project:{  
         questions:{  
            $filter:{  
               input:"$questions",
               as:"qu",
               cond:{  
                  $eq:[  
                     "$$qu.order",
                     1
                  ]
               }
            }
         }
      }
   },
   {  
      $project:{  
         "questions.answers.points":0
      }
   }
])

这个输出：

{
    "_id" : 1,
    "questions" : [
        {
            "_id" : 100,
            "question" : "who is the eldest ?",
            "order" : 1,
            "answers" : [
                {
                    "_id" : 101,
                    "answer" : "Bart Simpson"
                },
                {
                    "_id" : 102,
                    "answer" : "Lisa Simpson"
                },
                {
                    "_id" : 103,
                    "answer" : "Maggie Simpson"
                }
            ]
        }
    ]
}