how to handel the Composite key in Asp .net web form

Question

i have a two table one is <strong>sale</strong> which has a one primary key <span style="color: #ff0000;">sale_id</span> and second table which name is <strong>saleDetails</strong> which has one primary key <span style="color: #ff0000;">sale_detail_id</span> and second is <span style="color: #ff0000;">sale_id</span> as a foreign key these both make the Composite key my question is that <span style="color: #ff0000;">sale_detail_id</span> is auto increment so what i can do with the sale_id