MongoDB $substr 从索引到索引

Question

数据如"abc abc abc (xyz) efg efg".

我正在尝试从字符串中提取 xyz。

尝试在 $project 部分执行此操作：

 $project:   {
        bracketextract : { $substr: [ "$title",{ $indexOfCP: [ "$title", "(" ]}, { $indexOfCP: [ "$title", ")" ]} - { $indexOfCP: [ "$title", "(" ]} ]}, 
    }

Answer 1

你应该使用 $subtract to the length (you're using - currently) and there should be 1 $add-ed 到起点

db.col.aggregate([
    {
        $project:   {
            bracketextract : { $substr: [ "$title", { $add: [ { $indexOfCP: [ "$title", "(" ]}, 1 ] } , { $subtract: [ { $indexOfCP: [ "$title", ")" ]}, { $add: [ { $indexOfCP: [ "$title", "(" ]}, 1 ] } ] } ]}, 
        }
    }
])

使用 $let 的可读性更高：

db.col.aggregate([
    {
        $project: {
            bracketextract: { 
                $let: {
                    vars: { 
                        start: { $add: [ { $indexOfCP: [ "$title", "(" ] }, 1 ] }, 
                        end: { $indexOfCP: [ "$title", ")" ] }
                    },
                    in: { $substr: [ "$title", "$$start", { $subtract: [ "$$end", "$$start" ] } ] }
                }
            }
        }
    }
])

MongoDB $substr 从索引到索引

MongoDB $substr from index to index

string

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mongodb