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Operating System:
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Steps To Reproduce:
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Run this code using mongo shell on an empty server.

This has the same results on server versions 4.0.6 and 4.1.8.

db.test.insert({ 'foo': { 'bar': 0, 'baz': 1 } }); for (projection of [ {_id: 0, foo: 1}, {_id: 0, foo: 1, 'foo.bar': 1}, {_id: 0, 'foo.bar': 1, foo: 1}, ]) { printjson(db.test.findOne({}, projection)); }

Output:

MongoDB shell version v4.0.0 connecting to: mongodb://127.0.0.1:27017 MongoDB server version: 4.0.6 { "foo" : { "bar" : 0, "baz" : 1 } } { "foo" : { "bar" : 0 } } { "foo" : { "bar" : 0, "baz" : 1 } }
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Run this code using mongo shell on an empty server. This has the same results on server versions 4.0.6 and 4.1.8. db.test.insert({ 'foo' : { 'bar' : 0, 'baz' : 1 } }); for (projection of [ {_id: 0, foo: 1}, {_id: 0, foo: 1, 'foo.bar' : 1}, {_id: 0, 'foo.bar' : 1, foo: 1}, ]) { printjson(db.test.findOne({}, projection)); } Output: MongoDB shell version v4.0.0 connecting to: mongodb: //127.0.0.1:27017 MongoDB server version: 4.0.6 { "foo" : { "bar" : 0, "baz" : 1 } } { "foo" : { "bar" : 0 } } { "foo" : { "bar" : 0, "baz" : 1 } }

When querying documents, the order of the keys in the projection object change the results when it contains both a key and its sub-key.

The reporter could not find documentation for this behavior, so it seems unintended.

The reporter would expect all these projections to be equal:

{foo: 1}
{foo: 1, 'foo.bar': 1}
{'foo.bar': 1, foo: 1}

duplicates

SERVER-6527 in projection implementation, dotted field inclusion supersedes parent field inclusion