MongoDB：如何在聚合上应用多个 $project

Question

下面给出的是来自 my_collection

{
  "_id":ObjectId("5b3f4e1013aad111501b34c4")
  "Text":["kjsdf sdfjgnks vk ekl sdsdd."],
  "Title":["lkdjf wufk"],
  "List":["3412","1244","7654","8476"],
  "__v":0
}

我想聚合my_collection这样的：

1. 生成一个字段“totalList”，它包含字段“List”中数组中的项目数。
2. 防止字段“__v”出现在结果中。
3. 将输出字段“列表”的值更改为数组的连接
4. 删除“列表”中字符串末尾的尾随逗号
5. 将第 4 点生成的字符串剪短，只包含“列表”字段中的前 50 个字符

所以我写了这段代码：

  db.my_collection.aggregate( 
      [ 
         {
           $addFields: {
             totalList: { $size: "$List" }  // I can achieve the 1st point with this
           }
         },
         { $project : { 
                          __v:0,  // I can achieve the 2nd point with this
                          List:
                          {
                            $reduce:  // I can achieve the 3rd point with this
                               {
                                 input: "$List",
                                 initialValue: "",
                                 in: { $concat : ["$$value", "$$this", ","] }
                               }
                          }
                       }
          }
      ] 
  );

但是，当我运行上述命令时出现以下错误：

  Failed to execute a script.

  Error:
  Assert: command failed: {
    "ok" : 0,
    "errmsg" : "Bad projection specification, cannot include fields or add computed fields during an exclusion projection: { __v: 0.0, List: { $reduce: { input: \"$List\", initialValue: \"\", in: { $concat: [ \"$$value\", \"$$this\", \",\" ] } } } }",
    "code" : 40182,
    "codeName" : "Location40182"
  } : aggregate failed
  _getErrorWithCode@src/mongo/shell/utils.js:25:13
  doassert@src/mongo/shell/assert.js:16:14
  assert.commandWorked@src/mongo/shell/assert.js:370:5
  DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1319:5
  DBCollection.prototype.aggregate@:1:355
  @(shell):1:1

  Error: command failed: {
    "ok" : 0,
    "errmsg" : "Bad projection specification, cannot include fields or add computed fields during an exclusion projection: { __v: 0.0, List: { $reduce: { input: \"$List\", initialValue: \"\", in: { $concat: [ \"$$value\", \"$$this\", \",\" ] } } } }",
    "code" : 40182,
    "codeName" : "Location40182"
  } : aggregate failed :
  _getErrorWithCode@src/mongo/shell/utils.js:25:13
  doassert@src/mongo/shell/assert.js:16:14
  assert.commandWorked@src/mongo/shell/assert.js:370:5
  DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1319:5
  DBCollection.prototype.aggregate@:1:355
  @(shell):1:1

因此，我将其改写为：

  db.my_collection.aggregate( 
      [ 
         {
           $addFields: {
             totalList: { $size: "$List" }  // Achieve the 1st point with this
           }
         },
         { $project : { 
                          "Text":1,
                          "Title":1,
                          __v:{     // Achieve the 2nd point with this
                              $cond: {
                                 if: { $eq: [1,1] },
                                 then: "$$REMOVE",
                                 else: 0
                              }
                          },
                          List:
                          {
                            $reduce:  // Achieve the 3rd point with this
                               {
                                 input: "$List",
                                 initialValue: "",
                                 in: { $concat : ["$$value", "$$this", ","] }
                               }
                          }
                       }
          }
      ] 
  );

现在，我可以达到前 3 分。
但是，我怎样才能达到第 4 点和第 5 点？

Answer 1

您可以尝试以下聚合

db.collection.aggregate([
  { "$addFields": {
    "totalList": { "$size": "$List" },
    "List": {
      "$substr": [
        { "$reduce": {
          "input": "$List",
          "initialValue": "",
          "in": { "$concat": ["$$value", ",", "$$this"] }
        }},
        1,
        50
      ]
    }
  }},
  { "$project": { "List": 1, "Test": 1, "Title": 1, "totalList": 1 }}
])

输出

[
  {
    "List": "3412,1244,7654,8476",
    "Title": [
      "lkdjf wufk"
    ],
    "_id": ObjectId("5b3f4e1013aad111501b34c4"),
    "totalList": 4
  }
]

在这里试试