Aggregations: Use $cond without an else clause

How can I build a condition without an else clause like this in a $set stage?

{
	$set: {
		isLastReceiveTime: {
			$cond: {
				if: { 
					"$eq": ["$last", true] 
				}, 
				then: "$$NOW"
				// No else clause
			}
		}
	}
}

That is if the condition resolves to true “isLastReceiveTime” is set, otherwise it is untouched.
I found the suggestion to use an else clause like this:

{
	$set: {
		isLastReceiveTime: {
			$cond: {
				if: { 
					"$eq": ["$last", true] 
				}, 
				then: "$$NOW" ,
				else: "$$REMOVE"
			}
		}
	}
}

but this would remove an possibly already existing value in the isLastReceiveTime what is not what I want.

A workaround is to first store a possibly existing value of “isLastReceiveTime” in a temporary field and then use this in the else clause:

{
	$set: {
		isLastReceiveTimeBack: "$isLastReceiveTime" 
		isLastReceiveTime: {
			$cond: {
				if: { 
					"$eq": ["$last", true] 
				}, 
				then: "$$NOW" ,
				else: "$isLastReceiveTimeBack"
			}
		}
	}
}

But this is a more than awkward way and would require to delete the new “isLastReceiveTimeBack” field in a later stage …

Hi @Craig_Crevola ,

this is very nice, thanks a lot.
I didn’t have that on my radar …