从子文档返回字段数组

Question

谁能帮我查询 mongo 来展开数组，我正在使用 mongdb 本地驱动程序我的集合文档如下，另外请忽略我的 objectId 它只是为了示例

{  
      "_id":ObjectId(123),
      "name":"Sam",
      "age":20,
      "hobbiesDetail":[  
         {  
            "description":"FootBall",
            "level":"20%"
         },
         {  
            "description":"Cricket",
            "level":"80%"
         }
      ]
   },
   {  
      "_id":ObjectId(124),
      "name":"Ted",
      "age":26,
      "hobbiesDetail":[  
         {  
            "description":"FootBall",
            "level":"20%"
         }
      ]
   }

我的预期输出是

[  
   {  
      "name":"Sam",
      "age":20,
      "hobbies":"Football,Cricket"
   },
   {  
      "name":"Ted",
      "age":26,
      "hobbies":"Football"
   }
]

我只是想展开我的数组并在一个查询中在爱好描述之间添加一个逗号，感谢任何帮助

Answer 1

您只需要一个带有$map 数组运算符的$projection。

db.collection.aggregate([
    { "$project": { 
        "name": 1, 
        "age": 1, 
        "hobbies": { 
            "$map": { 
                "input": "$hobbiesDetail", 
                "as": "hobby", 
                "in": "$$hobby.description" 
            } 
        } 
    }}
])