Conditional grouping with $exists inside $cond

Question

I have two keys A and B and their existence in the document is mutually exclusive. I have to group by A when A exists and group by B when B exists. So I am $project

Answer 1

if one wants to check $exists with in $cond an alternative approach is to use $not with $cond

{$project: {MyKey: {$cond: [{$not: ["$A"]}, "$B", "$A"]}}} 

and truth table for $not is as

Hopes that Helps