saving array into the field of an array cdt

Question

I have a rule input of type cdt array. What I want to do is to add a list of values to one of the fields by looping through another list, something like in the example below. 
 a!save( ri!test.user_name, a!forEach( items: {1,2}, expression: "hey" ) ), 
 the problem is that it adds all the values to all the rows in the cdt, so it looks like this 
 {{ user_name: hey;hey}, {user_name: hey;hey}} 
 I've done this many times and never had this issue before. What am I doing wrong? 
 doing save( ri!test.user_name, {"hey", "hey}) returns the same output. So I don't know what's happening. 
 Any ideas on doing this?

Mike Schmitt · Answer

I'm a little unclear on some assumptions you're making here - for example where does the "items: {1, 2}" list come from? 
 What you have already written will save, over the entire array of ri!test, a list of "hey" text values for every "item" you iterate over. Apparently this saves this into each member of the array. I wouldn't have been sure without testing, but I'm not terribly surprised - Appian does certain array/set operations a bit unintuitively sometimes. 
 So instead, if we assume that your ri!test is starting with 2 members of the array and you want to save the value "hey" into both username fields, then you would so something more like 
 saveInto: {
 a!forEach(
 ri!test,
 a!save(
 fv!item.user_name,
 "hey " & fv!index /* this would make the user_name populate with "hey 1", "hey 2", etc */
 )
 )
}

Mike Schmitt · Answer

if the CDT array is starting out empty, you will need to do something to instantiate its size, regardless of what else you do I think. I mean, you could always manually construct a JSON string that would translate into roughly your CDT dictionary and then save that result back into the CDT, but I don't see that significantly deviating from the construct of looping over a constructor. 
 Note, I think you don't have to actually call the "type!" constructor itself for each item in the loop - you would need only set the property that agrees with the CDT property you want; the rest would be taken care of by the typecasting that happens automatically during the save. If that's still not sufficient, then I'm kinda out of ideas.