Set FileMaker Field Using Variable for Field Name

-- Demo script to roll back a data table from a transaction log
-- The script works on a file with 2 tables, "Log" and "Data"
-- Start with 2 windows open

tell  application "FileMaker Developer"
   activate -- bring FileMaker to front
   go to  document "SetDataFromLog.fp7" -- bring database file to front
   tell  document "SetDataFromLog.fp7" -- document = found set
      tell  window 1 to  go to  layout "Data"
      tell  window 2 to  go to  layout "Log"
      tell  window 2 -- Log
         -- Extract the data from the log table into 3 arrays
         tell  layout "Log"
            set  the_field_names  to  field "FieldName"
            set  the_field_values  to  field "Value"
            set  the_IDs  to  field "ID_Contact"
         end  tell
      end  tell
      set  the_count  to  count  of  the_field_names
   end  tell
end  tell

tell  application "FileMaker Developer"
   tell  database "SetDataFromLog.fp7" -- database is the entire database
      -- Loop through the arrays that were created earlier
      repeat  with  i  from 1 to  the_count
         -- Extract the data from the 3 arrays that were created earlier
         set  a_field_name  to  item  i  of  the_field_names
         set  a_field_value  to  item  i  of  the_field_values
         set  an_ID  to  item  i  of  the_IDs  as  string
         show (every  record  whose  cell "ID" is  an_ID) -- use key to find record to update
         delay 1 -- second -- slow script for the demo
         -- The next line sets a field base on a variable
         set  cell  a_field_name  of  current record  of  window 1 to  a_field_value
         delay 1
      end  repeat
   end  tell
end  tell