Question

Let’s say I have two tables: Book and Category. Book has a foreign key to Category. Book can only have on category. Let’s say I want to display this table:

Book Title              | Category ID       | Category Name
-------------------------------------------------------------------------
Lord of the Rings       | 1                 | Fiction
Ender's Game            | 2                 | Science Fiction
Purpose Driven Life     | 3                 | Religious

With domain model and data mapper (Fowler’s definition), we have Book and Category objects, each with their own mappers. The problem here is since both are separated entities, I end up with lots of queries run to display the above simple table:

$booksArray = $bookMapper->getTopTenBooks();
$viewData = array();

foreach ($booksArray as $book)
{
    $categoryID = $book->getCategoryID();
    $category = $categoryMapper->getByID($categoryID);
    $viewData[] = array(
        'book' => $book,
        'category' => $category
    );
}

return $view->load($viewData);

That’s one query to get the list of books to display, along with many queries to get the appropriate category objects related to each books.

The thing is, with SQL, I can slice the data any way I want. I can actually get all the data I need with one query:

SELECT
    book.name, category.name
FROM
    book LEFT JOIN category ON (book.cat_id = category.id)

Can I use this SQL while still maintaining separation between presentation, data source and domain logic layer? If I can, how? How can we effectively slice data we wanted to display?

Answer 1

I think that Mapper just should have Map() method to map array to object, then you will be able to use SQL directly via PDO and then just map result to objects.