用程序关联两个表，为啥表2不显示表1的数据呢？

各位大侠好，小弟正学习delphi，用程序关联两表的内容，现有两个表：
表一parts:
partnum  description   qtylnstock   sellingprice
表二items:
itemnum  custnum   partnum   qtysold   itempartnum
我现在要用表二的itempartnum字段显示表一的partnum字段的值，程序如下：
unit CItems;
interface
uses
  Windows, Messages, Classes, SysUtils, Graphics, Controls, StdCtrls, Forms,
  Dialogs, DBCtrls, DB, DBGrids, DBTables, Grids, ExtCtrls;
type
  TForm2 = class(TForm)
    ItemsTableItemNum: TAutoIncField;
    ItemsTableCustNum: TFloatField;
    ItemsTablePartNum: TFloatField;
    ItemsTableQtySold: TFloatField;
    DBGrid1: TDBGrid;
    DBNavigator: TDBNavigator;
    Panel1: TPanel;
    DataSource1: TDataSource;
    Panel2: TPanel;
    ItemsTable: TTable;
    BtnExit: TButton;
    PartsTable: TTable;
    DataSource2: TDataSource;
    DBGrid2: TDBGrid;
    PartsTablePartNum: TFloatField;
    PartsTableDescription: TStringField;
    PartsTableQtyInStock: TFloatField;
    PartsTableSellingPrice: TFloatField;
    ItemsTableItemPartNum: TStringField;
    procedure FormCreate(Sender: TObject);
    procedure BtnExitClick(Sender: TObject);
    procedure ItemsTableCalcFields(DataSet: TDataSet);
  private
    { private declarations }
  public
    { public declarations }
  end;
var
  Form2: TForm2;
implementation
{$R *.DFM}
procedure TForm2.FormCreate(Sender: TObject);
begin
  ItemsTable.Open;
end;
procedure TForm2.BtnExitClick(Sender: TObject);
begin
  Application.Terminate;
end;
procedure TForm2.ItemsTableCalcFields(DataSet: TDataSet);
var
FindResultBoolean:Boolean;
begin
   FindResultBoolean:=PartsTable.FindKey([ItemsTablePartNum]);
    if  FindResultBoolean then
      ItemsTableItemPartNum.Value:=PartsTableDescription.Value;
end;
end.
程序运行时报错：
还请高手指点！