Conditional grouping with $exists inside $cond

problem ：

I have two keys A and B and their existence in the document is mutually exclusive. I have two keys A and B, Their existence in the document is mutually exclusive .I have to group by A when A exists and group by B when B exists. When A I must press... When I exist A grouping , When B I must press... When I exist B grouping .So I am $project ing the required value into a computed key called MyKey on which I'll perform a $group . So I $project Put the required values into a file named MyKey In the calculation key of , I will execute... On this key $group .But it looks like I'm making a mistake with the syntax. But it seems that I made a mistake in grammar .I tried writing $project in two ways: I try to write in two ways $project ：

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

and and

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

But I keep getting the error: But I keep getting error messages ：

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

What's going wrong? What's up? ？

---

Solution ：

Reference resources ： https://stackoom.com/en/question/xdcC