How to create a collection within a collection

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Greeting all,

I have a data structure like the following:

1 Request

  • id
  • department
  • invoicesDetails
    • invoice 1
      • amount
      • tax
    • invoice 2
      • amount
      • tax
    • invoice 3
      • amount
      • tax

2 Request

  • id
  • department
  • invoicesDetails
    • invoice 1
      • amount
      • tax
    • invoice 2
      • amount
      • tax

I was able to make a collection of objects,
but could not figure out how to do a collection inside a collection and access it.
I can’t add a collection to an item “error”

I tried to use Datatable and set a column as a collection, but again I couldn’t access the data.

RequestsData.Rows(0).item(“invoicesDetails”)
I don’t know how to access it, for each doesn’t work here

Could you help with the best solution for this scenario?

Thank you a lot.