OPERATIONAL DEFECT DATABASE
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> db.foo.drop() db.aggregate([{$documents:[{z: 1, a: 1}]}, {$merge: {into: "foo", whenMatched:"keepExisting", whenNotMatched:"insert"}}]) db.foo.find() When I run the above I get: [ { _id: ObjectId('68baf740fcc543022431a453'), a: 1, z: 1 } ] Note that `find` shows the `a` field first, even though the aggregation has `z` first. Per max.hirschhorn@mongodb.com, this bug is specific to this part of whenMatched/whenNotMatched settings. It does not happen, for example, if `whenMatched` is `replace`. Workaround whenMatched: [{$replaceWith: "$$ROOT"}]
max.hirschhorn@10gen.com commented on Fri, 5 Sep 2025 15:37:44 +0000: The reason why I consider this a bug is because we have a parameter value named "insert" in $merge and SERVER-74408 acknowledges that insertOne() and replaceOne() will preserve field order.
See above.
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