Today I created a form and needed to work out how to use the "RSForms Pro Plugin - Joomla! Articles" to populate a Subform Custom Field row.

Solution:
Create the value being placed in the subform field as a JSON string so that it populates the subform field in the database.

How to know what the format needs to be?

• You need to go and look in your #__fields_values table to see what the JSON string is for an existing record.
• The table does not have a primary key, so you might have trouble seeing the full string. To get the full string, I used Regular Labs DB Replacer and did a search for values in that table were the field_id matches the subform field ID.
• Copy that string and place it in the mapping in your RSForm
• Replace the existing values in the copied string with the placeholder values.
• Save and test your form.

Note that you don't need to additionally map each individual field in the subform - just the single record.

So that's the basics to get a single subform record to be in place.

Multiple subform rows
RSForms Pro at the moment doesn't have a built in or plugin based tool to manage multiple subform records at the moment. So if you need to collect multiple rows you may be able to achieve that by:

• Creating repeating rows with a conditional fields configuration.
• Mapping sets of those fields to additional JSON strings - note that this is difficult to do currently without ending up with blank rows.
• You'll need to work out some PHP to do it.
• Put a feature request in to RSJoomla to add the related functionality to the plugin.