$let inside $map not working properly

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    • Type: Bug
    • Resolution: Done
    • Priority: Major - P3
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    • Affects Version/s: 3.6.0
    • Component/s: Aggregation Framework
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      Given the following document:

      db.test.insertOne({
    "_id" : ObjectId("5b27d2c5dd24433254a066a6"),
    "input" : "ab"
})

      When I run:

       

      db.test.aggregate({
    $project: {
        "result": {
            $map: {
                input: { $range: [ 0, { $strLenCP: "$input" } ] }, // array: 0 to number of chars in input string
                in: { 
                    $let: {
                        vars: { "char": { $substrCP: [ "$input", "$$this", 1 ] } }, // nth character will be stored in "$$char"
                        in: {
                            $switch: {
                                branches: [{
                                    case: { $eq: [ "$$char", "a" ] }, then: 1, // this should get matched
                                    case: { $eq: [ "$$char", "b" ] }, then: 2 // but only this one does get matched
                                }],
                                default: "$$char"
                            }
                        }
                    }
                }
            }
        } 
    }
})

      I would expect to receive:

       

      {
    "_id" : ObjectId("5b27d2c5dd24433254a066a6"), 
    "result" : [ 
        1.0, 
        2.0
    ]
}

      However, the result is this:

      {
    "_id" : ObjectId("5b27d2c5dd24433254a066a6"),
    "result" : [ 
        "a", 
        2.0
    ]
}

       

       

      Show
          Given the following document: db.test.insertOne({ "_id" : ObjectId( "5b27d2c5dd24433254a066a6" ), "input" : "ab" }) When I run:   db.test.aggregate({ $project: { "result" : { $map: { input: { $range: [ 0, { $strLenCP: "$input" } ] }, // array: 0 to number of chars in input string in: { $let: { vars: { " char " : { $substrCP: [ "$input" , "$$ this " , 1 ] } }, // nth character will be stored in "$$ char " in: { $ switch : { branches: [{ case : { $eq: [ "$$ char " , "a" ] }, then: 1, // this should get matched case : { $eq: [ "$$ char " , "b" ] }, then: 2 // but only this one does get matched }], default : "$$ char " } } } } } } } }) I would expect to receive:   { "_id" : ObjectId( "5b27d2c5dd24433254a066a6" ), "result" : [ 1.0, 2.0 ] } However, the result is this: { "_id" : ObjectId( "5b27d2c5dd24433254a066a6" ), "result" : [ "a" , 2.0 ] }    
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      In v3.6.0, $let inside $map appears to get evaluated at the wrong point in time. See "Steps To Reproduce" for details.

       

            Assignee:
            Asya Kamsky
            Reporter:
            Daniel Hegener
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              Created:
              Updated:
              Resolved: