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

Use $ifNull instead of $cond in your $project:

{ $project: {MyKey: {$ifNull: ['$A', '$B'] }}}

If A exists and is not null its value will be used; otherwise the value of B is used.