Question

数据：

{code: "XXXXXXXX1", total: 400},
{code: "YYYYY2", total: 500}
{code: "ZZZZZZ3", total: 100}
{code: "AAA5", totala: 200}

我想创建一个聚合函数，按照code字段中的最后一个字符对上面的数据进行分组。 code字段是一个字符串，长度可以变化。我只想得到它的最后一个索引/数字。类似的东西：

db.transactions.aggregate([
 {$project: {
   last_index: {$getMyLastCharInMyCode: "$code"},
   total: 1
 }},
 {$group: {_id: "$last_index", {total: {$sum: "$total"}}}}
])

我搜索了互联网和mongodb手册，似乎不可能。有任何想法吗？谢谢

Answer 1

你去了：

db.transactions.aggregate({
    $addFields: {
        "last_index": { $substr: [ "$code", { $subtract: [ { $strLenCP: "$code" }, 1 ] }, 1 ] }
    }
})

Answer 2

db.transactions.aggregate([
  {"$project": {
         total: 1,
         code: 1,  
         last_index: { $substr: [ "$code", { $subtract: [ {"$strLenCP": "$code"}, 1 ] }, -1 ]}
      }
 },
 {"$group": {
       "_id": "$last_index", 
       "total": {"$sum": "$total"}
     }
  }
])

如何从mongodb中的字符串中获取最后一个字符？

2 个答案: