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 …
