Is there a straight forward way to save to an individual field within a multi-va

...unctions like an apply. I imagine this would be a relatively common problem, anyone with a good supported solution/workaround? I know this can be achieved in the process model but my specific use case requires that I do it in SAIL.

The trick is to create the array on the right hand side and save into the parent level. You can try using a type constructor as follows:
a!save(local!customers, apply(type!CUSTOMERS_CDT(id: _), 1+enumerate(length(local!customers)))

Thanks Tim. I already had this in the back of my mind but I wanted to keep this as a last option. The main issue I've with this is that in addition to populating the ID field I also have to copy over all the other customer data field by field. This becomes an issue whereby whenever new fields are added to the customer CDT, this piece of code will also have to be updated to copy over those new fields and this can be easily missed.

As an aside can I ask why you're manually creating the customer ids rather than having the rdbms do it?

I think there is a plugin to update certain fields of a CDT. This in combination with apply should let you do what you want.

forum.appian.com/.../summary

This plugin definitely looks like it might help, specifically the updateCDT function - will try it out and see. Was hoping that I was missing something simple and Appian would support this natively though!
@tim, good point about leaving ID generation to the DB but my use case is actually not about updating customer IDs, I just thought this example would be easier to highlight the issue - I've several complex use cases where a user action on a form results in the need to re-evaluate certain fields in list of entities all of which are multi-value CDTs.

The plugin works but with some quirks. I was able to make it work by externalizing the code below is a separate rule and calling that rule within an apply function:
/*rule!updateCustomerID*/
updateCDT(ri!customer, {id: ri!id})
/*rule!updateCustomerIDs*/
apply(rule!updateCustomerID(_, _), merge(local!customers, enumerate(length(local!customers)) + 1))

One issue I observed is that the plugin function fails with an exception when trying to clear a field(set to null):
updateCDT(ri!customer, {id: null}) - this will always fail with an exception
updateCDT(ri!customer, {id: ""}) - this seems to work but makes it hard to use this in a dynamic way with a functional that can return null and non-null values at run time

Also this uses Appian dictionaries making it quite difficult to make this code dynamic and generic since this requires being able to dynamically create dictionary keys which is not very well-supported. I was able to do this is a pretty fragile way using a recently introduced json function: jsontodictionary("{"""&tostring(ri!field)&""": "&ri!value&"}")!

Haven't decided which way to go yet but at least I've several options...

The plugin contains the source code, so with a bit of Java knowledge you should be able to adapt it to your needs.

Yeah I agree, some simple tweaking can make this fit my use case perfectly. Will probably end up doing that.