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  1. Core Server
  2. SERVER-50614

JS 'Set' type does not serialize well to BSON

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Details

    • Improvement
    • Status: Backlog
    • Major - P3
    • Resolution: Unresolved
    • None
    • None
    • JavaScript

    Description

      As you can see below - it just happens to return an empty object, but proceeds without error. It seems better to either (a) error or (b) return an array. We don't have a 'set' data type in the BSON spec, so I don't see how we could get it to work and round trip seamlessly, but this experience isn't great.

      > db.foo.aggregate([{
          $project: {
              js_res: {
                  $function: {
                      lang: "js",
                      body: function() {
                          return new Set([1, 2]);
                      },
                      args: []
                  }
              }
          }
      }])
      { "_id" : ObjectId("5f3c24f10efd1cb7f0a88f4a"), "js_res" : {  } }
      { "_id" : ObjectId("5f3c24f60efd1cb7f0a88f4b"), "js_res" : {  } }
      { "_id" : ObjectId("5f47f9411945d1527c6b0368"), "js_res" : {  } }
      { "_id" : ObjectId("5f47f9c71945d1527c6b0369"), "js_res" : {  } }
      { "_id" : ObjectId("5f47fa721945d1527c6b036a"), "js_res" : {  } }
      

      As a workaround, you can use the [...set] syntax to expand the set into an array:

      > db.foo.aggregate([{
          $project: {
              js_res: {
                  $function: {
                      lang: "js",
                      body: function() {
                          return [...new Set([1, 2])];
                      },
                      args: []
                  }
              }
          }
      }])
      { "_id" : ObjectId("5f3c24f10efd1cb7f0a88f4a"), "js_res" : [ 1, 2 ] }
      { "_id" : ObjectId("5f3c24f60efd1cb7f0a88f4b"), "js_res" : [ 1, 2 ] }
      { "_id" : ObjectId("5f47f9411945d1527c6b0368"), "js_res" : [ 1, 2 ] }
      { "_id" : ObjectId("5f47f9c71945d1527c6b0369"), "js_res" : [ 1, 2 ] }
      { "_id" : ObjectId("5f47fa721945d1527c6b036a"), "js_res" : [ 1, 2 ] }
      

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              backlog-query-optimization Backlog - Query Optimization
              charlie.swanson@mongodb.com Charlie Swanson
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                Created:
                Updated: