当并非所有的字段都有值时，联接字段

Question

我希望通过聚合管道中的投影阶段修改字段，该字段是由(-)分隔的其他字段值的组合。

如果字段为空或为空，则不会将其添加到协连字符串中。

{$project:{

//trial-1:
finalField:{
 $concat["$field1",'-','$field2','-','$field3',...] 
//problem1: $concat will return null if any of it's arguments is null or missing
//problem2: if all the fields are exist with non-null values, the delimiter will exists even if the field dosen't 
}


//trial-2:
finalField:{
 $concat:[
  {$cond:[{field1:null},'',{$concat:['$field1','-']},..]
//the problem: {field1:null} fails if the field dosen't exixt (i.e the expression gives true)

//trial-3
finalField:{
 $concat:[
  {$cond:[{$or:[{field1:null},{field:{$exists:true}},'',
   {$concat:['$field1','-']}
]}]}
]
}

] 
}

//trial-4 -> using $reduce instead of $concate (same issues)

}

Answer 1

你基本上想要$ifNull。它与$exists类似，但对于聚合语句，当字段表达式返回null时，它返回一个默认值，意思是“不在那里”：

{ "$project": {
  "finalField": {
    "$concat": [
      { "$ifNull": [ "$field1", "" ] },
      "-",
      { "$ifNull": [ "$field2", "" ] },
      "-",
      { "$ifNull": [ "$field3", "" ] }
    ]
  }
}}

例如，对于数据，如：

{ "field1": "a", "field2": "b", "field3": "c" },
{ "field1": "a", "field2": "b" },
{ "field1": "a", "field3": "c" }

当然，没有任何错误产生：

{ "finalField" : "a-b-c" }
{ "finalField" : "a-b-" }
{ "finalField" : "a--c" }

如果您想要更漂亮的东西，那么可以动态地使用名称，如下所示：

  { "$project": {
    "finalField": {
      "$reduce": {
        "input": {
          "$filter": {
            "input": { "$objectToArray": "$$ROOT" },
            "cond": { "$ne": [ "$$this.k", "_id" ] }
          }
        },
        "initialValue": "",
        "in": { 
          "$cond": {
            "if": { "$eq": [ "$$value", "" ] },
            "then": { "$concat": [ "$$value", "$$this.v" ] },
            "else": { "$concat": [ "$$value", "-", "$$this.v" ] }
          }
        }
      }
    }
  }}

它可以知道实际存在哪些字段，并且只尝试加入这些字段：

{ "finalField" : "a-b-c" }
{ "finalField" : "a-b" }
{ "finalField" : "a-c" }

如果不希望文档或子文档上的$objectToArray，甚至可以手动指定字段列表：

  { "$project": {
    "finalField": {
      "$reduce": {
        "input": {
          "$filter": {
            "input": ["$field1", "$field2", "$field3"],
            "cond": { "$ne": [ "$$this", null ] }
          }
        },
        "initialValue": "",
        "in": { 
          "$cond": {
            "if": { "$eq": [ "$$value", "" ] },
            "then": { "$concat": [ "$$value", "$$this" ] },
            "else": { "$concat": [ "$$value", "-", "$$this" ] }
          }
        }
      }
    }
  }}