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  1. Core Server
  2. SERVER-97149

$unwind with a path that traverses through arrays omits results

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Minor - P4 Minor - P4
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      The $unwind docs say: "When the operand does not resolve to an array, but is not missing, null, or an empty array, $unwind treats the operand as a single element array.". This should be a no-op. For example,

      MongoDB Enterprise > db.foo.insert({a: {b: 1}})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise > db.foo.aggregate({$unwind: "$a.b"})
{ "_id" : ObjectId("67361c61681dab8a44bc64f9"), "a" : { "b" : 1 } }

      But when the $unwind path is a nested path that traverses through arrays, those documents are omitted from the output:

      MongoDB Enterprise > db.foo.drop()
true
MongoDB Enterprise > db.foo.insert({a: [{b: 1}, {b: 2}]})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise > db.foo.insert({a: [{b: [1, 2]}]})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise > db.foo.aggregate({$unwind: "$a.b"})
MongoDB Enterprise >

            Assignee:
            alya.berciu@mongodb.com Alya Berciu
            Reporter:
            hana.pearlman@mongodb.com Hana Pearlman
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