mongodb - 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 $projecting the required value into a computed key called MyKey on which I'll perform a $group. But it looks like I'm making a mistake with the syntax. I tried writing $project in two ways:

{$project: {MyKey: {$cond: [{$exists: ["$A", true]}, "$A", "$B"]}}}

and

{$project: {MyKey: {$cond: [{"A": {$exists:true}}, "$A", "$B"]}}}

But I keep getting the error:

{ "errmsg" : "exception: invalid operator '$exists'", "code" : 15999, "ok" : 0 } ...

What's going wrong?

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.