I am trying to import purchase orders into odoo and have sorted out my problems for the most part however i am unable to update the list of products using the external ID. i can use the database ID or the name of the product however if i try to use the external ID i get this error:

• insert or update on table "purchase_order_line" violates foreign key constraint "purchase_order_line_product_id_fkey" DETAIL: Key (product_id)=(644) is not present in table "product_product". between rows 2 and 4
• 
  

Could someone please explain to me why this is happening and what i have to do to fix it?