MongoDB聚合变量来创建多个字段? [英] MongoDB Aggregate Variable to create Multiple Fields?

问题描述

给出以下查询，使用$$ priceToInflationRatio帮助创建多个计算字段的最佳方法是什么?根据我对$ let的了解，它似乎仅适用于创建单个字段-我想在我的整个$ project部分中使用变量.有可能吗?

Given the following query, what is the best method to use $$priceToInflationRatio to help create multiple calculated fields? From what I have read on $let, it appears to only work for creating a single field -- I would like to use the variables across my entire $project section. Is that possible?

db.books.aggregate([
  {$project: {
        'priceInflationresult': {
            $let: {
                vars: {
                    'priceToInflationRatio': {
                        $multiply: [{'$divide': [{'$subtract': ['$price', 1]}, 5]}, 10]
          }
        },
        in: {
            '$cond': [
            {'$gt': ['$price', 5]},
            {'$mod': ['$$priceToInflationRatio', 1]},
            1
          ]
        },
      }
    }
  }}
])

推荐答案

$let表达式的in部分是一个对象，因此它可以接受多个键，每个键都可以是一个用范围内的变量:

The in part of a $let expression is an object, so it can accept multiple keys, each of which can be an expression that is evaluated with the variables in scope:

> db.test.insert({ "_id" : 0, "a" : 1, "b" : 1 })
> db.test.aggregate([{
    "$project" : {
        "test" : {
            "$let" : {
                "vars" : {
                    "c" : 2,
                    "d" : 3
                },
                "in": {
                    "a" : { "$add" : ["$a", "$$c"] },
                    "b" : { "$add" : ["$b", "$$d"] }
                }
            }
        }
    }
}]);
{ "_id" : 0, "test" : { "a" : 3, "b" : 4 } }

请注意，这必将创建子文档，因为不允许使用顶级$ let表达式.您可以在另一个$ project阶段进行更改.

Note that this will necessarily create subdocuments as top-level $let expressions are not allowed. You can change this with another $project stage.

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