APPENDIX A n MORE ADVANCED FEATURES 247 n (Free web space)
APPENDIX A n MORE ADVANCED FEATURES 247 n MORE ADVANCED FEATURES 247 There are several dissatisfying aspects to the mapping in Listing A-13. First, rather than mapping our product table, we have mapped the link table. This makes sense when you consider that the primary key formed from the two columns of this table uniquely identifies a colored product, which the product table alone cannot do. Second, we are obliged to create a number of distinct objects to represent the class: the Product class itself, a class to represent the primary key (inevitable where a composite id occurs), a class to represent the other attributes of the product, and the Colorclass. Last, the use of the columns more than once within the mapping requires us to flag them so that they cannot be written this is a read-only mapping. Using a View Fortunately, most databases provide a simple mechanism for manipulating a schema so that it better matches the business requirements. A database view will allow you to put together a join that appears to be a table. By a suitable choice of columns from the existing tables, you can construct a view that is much easier to map (see Listing A-14). Listing A-14. A View on the Product Tables create view vwProduct (ProductKey,ColorKey,Id,SKU,ColorId) AS select p.id as ProductKey, c.id as ColorKey, p.id as Id, p.sku as SKU, c.id as ColorId from product p, product_color pc, color c where p.id = pc.product_id and pc.color_id = c.id; This view effectively reformats our table so that it has a correct (composite) primary key formed from the link table s two columns. It makes the SKU data available directly, and it retains the foreign key into the colortable. Listing A-15 is a much more natural mapping.
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