Checking is_required property of ms access table fields

Hi,
Im using the following code for checking the 'nullable' property of ms access table fields with name 'tablename' and field 'fieldname' passed as variables to the function and returning 'NOT NULL' string. The function should check if the field is a 'is_required' field type.
Unfortunately, Im not getting the desired result ie. Primary key fields dont return a 'NOT NULL'. Any idea why this might be? Any help will be much appreciated

function GetRequired(ADOConnection: TADOConnection; tablename:string; fieldname:string):string;
var
fieldreq: TADODataSet;
i: integer;
comparefield: integer;
output: boolean;
begin
  fieldreq := TADODataSet.Create(nil);
  fieldreq.Connection := ADOConnection;
try
ADOConnection.OpenSchema(siColumns,VarArrayOf([Unassigned,Unassigned,tablename,Unassigned]),EmptyParam,fieldreq);
 
for i := 0 to fieldreq.RecordCount-1 do
  begin
comparefield :=AnsiCompareStr(fieldreq.FieldByName('COLUMN_NAME').AsString, fieldname);
if comparefield = 0 then begin
output := fieldreq.FieldByName('IS_NULLABLE').AsBoolean;
  if output
    then fieldreq.Next
  else result := 'NOT NULL';
end
else
fieldreq.Next;
end;
finally
fieldreq.Free;
end;
end;

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