MongoDB嵌套组？

Question

我试图在mongodb中实现嵌套组查询，并且我试图通过添加外部组而陷入困境。鉴于以下（简化）数据文档：

{
  "timestamp" : ISODate(),
  "category" : "movies",
  "term" : "my movie"
}

我试图获得所有类别的列表，并且在类别中应该有最多的术语。我希望我的输出像这样：

[
 { category: "movies", 
   terms: [ { term: "movie 1", total: 5000 }, { term: "movie 2", total: 200 } ... ]
 },
 { category: "sports", 
   terms: [ { term: "football 1", total: 4000 }, { term: "tennis 2", total: 250 } ... ]
 },
]

我的内心团体＆＃39;如下所示，将获得所有类别的前5名：

db.collection.aggregate([
    { $match : { "timestamp": { $gt: ISODate("2014-08-27") } } },
    { $group : { _id :  "$term", total : { $sum : 1 } } },
    { $sort : { total : -1 } },
    { $limit: 5 }
]);

// Outputs:
{ "_id" : "movie 1", "total" : 943 }
{ "_id" : "movie 2", "total" : 752 }

我将如何实施＆＃39;外部组＆＃39;？

另外，有时上面的聚合]离子返回一个空值（并非所有文档都有一个术语值）。我如何忽略空值？

提前致谢

Answer 1

在这种情况下，您需要两组。第一组生成一个文档流，每个术语和类别包含一个文档：

 { $group : { 
      _id :  { 
        category: "$category",
        term: "$term",
      },
      total: { $sum : 1 } 
   }
 }

然后第二个组将所有具有相同术语的文档合并为一个，使用$push运算符将类别合并到一个数组中：

 { $group : { 
      _id :  "$_id.category",
      terms: { 
          $push: { 
              term:"$_id.term",
              total:"$total"
          }
      }
   }
 }

Answer 2

查询：

function findLongestWordLength(str) {
  let longestWord = "";
  let currentWord = "";
  
  //Move through the string letter-by-letter
  for (let i = 0; i < str.length; i++) {
    if (str.charAt(i) === " ") { //If we're at a space character
      if (currentWord.length > longestWord.length) longestWord = currentWord; //Check if that word was the longest
      currentWord = ""; //Reset the current word
    } else {
      currentWord += str.charAt(i); //Not at a space character, still building the current word
    }
  }
  if (currentWord > longestWord) longestWord = currentWord; //End of string - check current word once more
  return longestWord;
}

const longest = findLongestWordLength("The quick brown fox jumped over the lazy dog");
console.log(longest);

响应：

    db.getCollection('orders').aggregate([
    {$match:{
        tipo: {$regex:"[A-Z]+"}
        }
    },
    {$group:
        { 
            _id:{
                codigo:"1",
                tipo:"$tipo",
            },
            total:{$sum:1}
        }
    },
    {$group:
        {
            _id:"$_id.codigo",
            tipos:
            {
                $push:
                {
                    tipo:"$_id.tipo",
                    total:"$total"
                }
            },
            totalGeneral:{$sum:"$total"}

        }

    }


]);

}