Ignore missing array objects from JSON in Dataweave - json

Question

I am using Anypoint Studio 6.1 and Mule 3.8.1 and am mapping JSON to JSON in Dataweave. In the JSON mapping I have an optional field called "Channels" which contains a list of strings. When the field is not there I get a warning in Dataweave. How can I write the Dataweave code to ignore if its null?
Dataweave code:
%dw 1.0
%output application/json skipNullOn="everywhere"
---
payload map ((payload01 , indexOfPayload01) -> {
Channels: payload01.Channels map ((channel , indexOfAccessChannel) -> channel)
})
I have tried to use "when" and also the "?" selector modifier but cannot get the syntax right.
Thanks

Answer 1

You were right to use when and the ? operator. You just need to use parentheses to make sure they apply to the right things. Note that I am using $ as a shorthand for the payload01 parameter in your example.
%dw 1.0
%output application/json
---
payload map {
(Channels: $.Channels map (lower $)) when $.Channels?
}
If you didn't need to use map on the Channels array within each item, you could just allow the null to pass through:
payload map {
Channels: $.Channels
}
This would yield the following for input objects that don't contain a Channels field:
{
Channels: null
}
Adding the parentheses allows us to use when to determine whether the whole key/value pair (aka tuple) should be output:
payload map {
(Channels: $.Channels) when $.Channels?
}
Yielding:
{
}