$addFields may add new fields in a different order in certain cases

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    • Type: Bug
    • Resolution: Duplicate
    • Priority: Major - P3
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    • Affects Version/s: 4.2 Required
    • Component/s: Aggregation Framework
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      db.coll.drop()
db.coll.insert({_id:1, obj: {obj: {}}})
db.coll.aggregate([{$project: {“obj.obj.obj.str”: 1}},  {$addFields: {“obj.obj.array”:  ‘abc’, “obj.obj.obj.num”: ‘xyz’}}])  

{ “_id” : 1, “obj” : { “obj” : { “obj” : { “num” : “xyz” }, “array” : “abc” } } }
  
// Now we just add a $match stage, and the order of new fields changes.
db.coll.aggregate([{$match: {_id: 1}}, {$project: {“obj.obj.obj.str”: 1}},  {$addFields: {“obj.obj.array”:  ‘abc’, “obj.obj.obj.num”: ‘xyz’}}])  

{ “_id” : 1, “obj” : { “obj” : { “array” : “abc”, “obj” : { “num” : “xyz” } } }

       

       

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        db.coll.drop() db.coll.insert({_id:1, obj: {obj: {}}}) db.coll.aggregate([{$project: {“obj.obj.obj.str”: 1}},  {$addFields: {“obj.obj.array”:  ‘abc’, “obj.obj.obj.num”: ‘xyz’}}])   { “_id” : 1, “obj” : { “obj” : { “obj” : { “num” : “xyz” }, “array” : “abc” } } }   // Now we just add a $match stage, and the order of new fields changes. db.coll.aggregate([{$match: {_id: 1}}, {$project: {“obj.obj.obj.str”: 1}},  {$addFields: {“obj.obj.array”:  ‘abc’, “obj.obj.obj.num”: ‘xyz’}}])   { “_id” : 1, “obj” : { “obj” : { “array” : “abc”, “obj” : { “num” : “xyz” } } }    
    • Query 2020-06-29, Query 2020-07-13, Query 2020-07-27
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      Under certain conditions $addFields may add new fields to the output document in a different order, even when applied to the same input document.

       

      Note that this behaviour was changed in 4.4 when find and aggregate projections were unified, and the order of new fields is always the same as in $addFields specification.

            Assignee:
            Anton Korshunov
            Reporter:
            Anton Korshunov
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              Created:
              Updated:
              Resolved: