custom field in products not available in checkout/cart

I have added a custom field (units) to the product table.
All is well on the admin side - I can edit/save the "units" field ok.

On the catalog side, the "units" field is not available in controller/checkout/cart.php.
I have modified model/catalog/product.php by adding:
'units' => $query->row['units'],

in the if ($query->num_rows) {
return array(
section of the code.

The field is available when I drill down through the categories, but when I click on "Shopping Cart" I get the error:
"Undefined index: units .....catalog_controller_checkout_cart.php" (long message - I shortened it).

Obviously the "Undefined index" indicates the field is not available.
What am I missing?

Regards,
Bill

Can you pl. let us know how you added the filed in Admin section.

to solve your issue, you can add custom products field with this: http://www.marketinsg.com/custom-product-fields