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Type: Bug
Resolution: Fixed
Priority: Major - P3
Fix Version/s: 3.4.19, 3.6.10, 4.0.5, 4.1.4
Affects Version/s: 4.0.1
Component/s: Aggregation Framework
Labels:
None

Backwards Compatibility:
Minor Change
Operating System:
ALL
Backport Requested:

v4.0, v3.6, v3.4
Steps To Reproduce:
Hide

Collection

[ { "array": [ { "k": "field", "v": 1 }, { "k": "field", "v": 2 } ] } ]

Query

db.collection.aggregate([ { $project: { object: { $arrayToObject: "$array" } } }, { $project: { "object": "$object", "field": "$object.field" } } ])

Expected output

[ { "field": 1, "object": { "field": 1 } } ]

Actual output

[ { "field": 1, "object": { "field": 2 } } ]
Show
Collection [ { "array" : [ { "k" : "field" , "v" : 1 }, { "k" : "field" , "v" : 2 } ] } ] Query db.collection.aggregate([ { $project: { object: { $arrayToObject: "$array" } } }, { $project: { "object" : "$object" , "field" : "$object.field" } } ]) Expected output [ { "field" : 1, "object" : { "field" : 1 } } ] Actual output [ { "field" : 1, "object" : { "field" : 2 } } ]
Sprint:
Query 2018-10-22
Case:

Using $arrayToObject on an array of {k: ..., v: ...} objects where some k are repeating outputs an object that behaves inconsistently.

Referencing this object as a whole using "$object" outputs an object with a field value that is different compared to the value obtained by referencing the field directly using "$object.field".