Azure Workbook parameter for resource and resource group - azure-monitoring

Question

I've been defeated by Kusto on what I thought to be a simple query...
I'm making my first workbook and playing with parameters. I can list and select a Resource Group, but I can't make the following parameter (Virtual Machines) populate with the VMs when more than one Resource Group is selected. The ResourceGroup passes a comma delineated string of the group names as a property resourcegroup just fine. I cannot figure out how to translate that string into a usable where-statement. My query works just fine when I manually string several Resource Groups together so I assume I'm getting burned by my understanding of let and arrays in Kusto. If there is a better way of doing what I'm trying to do, please let me know.
//This will work so long as 1 Resource Group is passed from the previous parameter
resources
| where resourceGroup in ('{ResourceGroup:resourcegroup}') and type =~ microsoft.compute/virtualmachines'
| project value = id , label = name
I've figured out I can get a proper array with split('{ResourceGroup:resourcegroup}',","), but, again, I haven't been able to marry up that object with a where-statement.
Any help is much appreciated!

Answer 1

in https://github.com/microsoft/Application-Insights-Workbooks/blob/master/Documentation/Parameters/DropDown.md#special-casing-all
NORMALLY there is a way to do this:
let resourceGroups = dynamic([{ResourceGroup:resourcegroup}]);// turns even an empty string into a valid array
resources
| where (array_length(resourceGroups)==0 // allows 0 length array to be "all"
or resourceGroup in (resourceGroups)) // or filters to only those in the set
and type =~ microsoft.compute/virtualmachines'
| project value = id , label = name
however, i don't think Azure Resource Graph allows using let this way?
if you get an error that let isn't allowed, you'll have to do that dynamic thing inline a couple times instead:
resources
| where (array_length(dynamic([{ResourceGroup:resourcegroup}]))==0 // allows 0 length array to be "all"
or resourceGroup in (dynamic([{ResourceGroup:resourcegroup}]))) // or filters to only those in the set
and type =~ microsoft.compute/virtualmachines'
| project value = id , label = name