Form element names with multiple new rows

I have a report with many rows some of which refer to an existing
ActiveRecord model object and some new ones. I’m trying to figure out
a naming convention for the form elements so that Rails will properly
convert it into a params hash in my controller.

My first idea is to do something like the below but Rails will confuse
the data for the two ‘new’ rows
Enter a new invitation

Enter a new invitation

Update an existing invitation

My second idea is to do something like the below and parse out the
‘new_’ params in my controller
Enter a new invitation


Enter a new invitation


Update an existing invitation

Is this a common problem that others have solved in a standard way
before? Any pointers to plugins or techniques others have used?

Thanks
Alex