add expression to get value by keyname from object

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    • Query 2020-08-10, Query 2020-08-24, Query Optimization 2021-04-05, Query Optimization 2021-04-19, Query Optimization 2021-05-03
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      While it's possible via combined use of $objectToArray and $filter with $map, it would be nice to have a simple expression to get value of particular field of an object:

      db.foo.find()
{ _id: 1, a: 5, foo: { b: "a", c: 99.9 } }
db.foo.aggregate({$project:{ foo_c_is: {$getField:[ "c", "$foo" ] } } })
{ _id: 1, foo_c_is: 99.9 }

      This will allow easily plucking out a value when the field name is stored in another part of the same document, or when it has some funny character that precludes it being used as subfield name (like leading "$") as in SERVER-30365 and SERVER-14466 or embedded '.' when querying oplog or system.profile collections.

      Second argument could be optional and default to "$$ROOT".

            Assignee:
            Alya Berciu
            Reporter:
            Asya Kamsky
            Votes:
            14 Vote for this issue
            Watchers:
            21 Start watching this issue

              Created:
              Updated:
              Resolved: