Update text Field into "GeneralJournalAccountEntry" table with "VendInvoiceJour" Invoice Description text value

Hi,
I need to transfer the Invoice Description text field after Posting a Purchase Invoice. I tried to include my code into the Insert, write, update methods on the GeneralJournalAccountEntry with no luck; After putting breakpoints and Using debugger i found that the system is not going through this methods.
is there any class that i should use to fill the the text field in GeneralJournalAccountEntry table..
Please i need a solution for this issue..