How to implement array of nested arrays

Question

Hi all, 
 I have to implement one requirement as shown below screenshots, I have a button called "add period details" then a form should be will be displayed to user and add the period details. This should be an parent array. In the same form there should be another sub-button called "Add Arm Titles" this should be a nested child CDT. which should be used to add multiple rows and these values need to be stored into database. Also the same way need to have the delete functionality, when user deletes a period details, the child rows "Arm Titles" should also get deleted. 
 
 Add Period Details -->Parent (period details) --> Multiple Child (add arm details) 
 can you please help us, how can we achieve this type of implementation, Quick responses on this thread are highly appreciated. 
 
 UI Screens CDT structure ---------------------

Stefan Helzle · Answer

Use the data type constructors to create the parent element and populate the nested field with a list of children. Seems to be a simple task. Where are you stuck?

Shubham Aware · Answer

I am not sure what issue you are facing(Help us with your actual problem in design) but quick suggestion.
Appian recommends only using nested CDTs for one-to-one and many-to-one relationships due to performance issues.
I would recommend use flat design with two separate CDTs where ArmTitles contains a foreign key (periodId) referencing PeriodDetails, implement two separate editable grids instead of nested forms, and handle the parent-child relationship through queries and process logic rather than CDT nesting.