can we hav an aggregation operator like "$arrayElemAt" 
but instead of working on array the input is a document

{ $elemAt: [ <document>, <key> ] }

The <document> expression can be any valid expression as long as it resolves to a document.

The <key> expression can be any valid expression as long as it resolves to a String.

 

Behavio

Example     ->     Results

{ $elemAt: [ \{k1:"v1",k2:"v2"}

, "k1" ] }   -> "v1"

{ $elemAt: [ \{k1:"v1",k2:"v2"}

, "k3" ] }   -> null