问题描述
我有两个键A和B，并且它们在文档中的存在是互斥的.我必须在A存在时按A分组，而在B存在时按B分组.因此，我将所需的值$project放入一个名为MyKey的计算键中，在该键上将执行$group.但似乎我在语法上犯了一个错误.我尝试通过两种方式编写$ project:
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", "$B"]}}}
{$project: {MyKey: {$cond: [{$exists: ["$A", true]}, "$A", "$B"]}}}
和
{$project: {MyKey: {$cond: [{"A": {$exists:true}}, "$A", "$B"]}}}
但我不断收到错误消息:
But I keep getting the error:
{ "errmsg" : "exception: invalid operator '$exists'", "code" : 15999, "ok" : 0 } ...
怎么了?
推荐答案
使用 $ifNull 而不是$project中的$cond:
{ $project: {MyKey: {$ifNull: ['$A', '$B'] }}}
如果A存在，但不存在，则将使用其值；否则，将使用B的值.
If A exists and is not null its value will be used; otherwise the value of B is used.
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