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Frank_Numrich1Frank Numrich
May 2024

In an aggregation stage I want to set several fields in case a condition results to true like this:

{
	$set: {
		$cond: {
		if: {
			$ne: ["$isLastReceiveTime", null]
		},	
			then: {
				complete: true,
				completedTime: "$$NOW"
			},
			else: {
				complete: "$$REMOVE",
				completedTime: "$$REMOVE"
			}	
		}
	}
}

But that does not work since $set expects a field name and not an operator like $cond …

Currently I’m doing it this way:

{
  complete: {
    $cond: {
        if: {
	   $ne: ["$isLastReceiveTime", null]
	},	
        then: true,
        else: "$$REMOVE"
    }
  },
  completedTime: {
    $cond: {
        if: {
	   $ne: ["$isLastReceiveTime", null]
	},	
        then: "$$NOW",
        else: "$$REMOVE"
    }
  }
}

But this is really not nice and maintainable …
turivishalVishal TuriMongoDB User Group Leader
May 2024

Hi @Frank_Numrich1, Welcome back,

I think there is no any better approach, but below is the same just a short-hand approach of the $cond operator.

{
    $set: {
      completedTime: {
        $cond: ["$isLastReceiveTime", "$$NOW", "$$REMOVE"]
      },
      complete: {
        $cond: ["$isLastReceiveTime", true, "$$REMOVE"]
      }
    }
  }
Frank_Numrich1Frank Numrich
May 2024

Hi @turivishal ,

that’s what I feared - so I have to stay with this …
In fact I already knew the short form but wasn’t aware that I don’t need the $ne statement which makes it much more readable.

Thanks and best regards,
Frank

Frank_Numrich1Frank Numrich
May 2024

In fact my condition is slightly more complex than in my example which I made easy to have the focus on the question …

The real statement looks like this:

{
  complete: {
    $cond: [{$and: ["$isLastReceiveTime", {$eq: [{$add: [{$max: "$mjrCounter"}, 1]}, {$size: "$mjrCounter"}]}]}, true, "$$REMOVE"]
  },
  completedTime: {
    $cond: [{$and: ["$isLastReceiveTime", {$eq: [{$add: [{$max: "$mjrCounter"}, 1]}, {$size: "$mjrCounter"}]}]}, "$$NOW", "$$REMOVE"]
  }
}

It surprises me that it is not possible to write the statement this way:

if <condition>
   <action_1>
   <action_2>

but instead have to write

<action_1>
   if <condition>
<action_2>
   if <condition>

No one would write code this way …

5 months later
Juan_Vidal_Pich
Sep 2024

The problem is not exactly the same because my need to set several fields conditionally was one level deep, I found that $mergeObjects would do the trick:

{
	$set: {
		myProp: {
			$mergeObjects:[
				"$myProp",
				$cond: [
					$ne: ["$isLastReceiveTime", null],
					{
						complete: true,
						completedTime: "$$NOW"
					},
					{
						complete: "$$REMOVE",
						completedTime: "$$REMOVE"
					}
				]
			]
		}
	}
}

For your use case probably a combination of $replaceRoot and $$ROOT would achieve the same effect.


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