Because of the problem described in ZF-2546, joins will fail on a Zend_Db_Table_Select:

$categoryTable = new CategoryTable(array(
                                       'db'      => 'db',
                                       'name'    => 'category',
                                       'primary' => 'id'));
// Get categories belonging to a product--
// A many-to-many relationship
$select = $categoryTable->select()
    ->join('category_product',
           'category_product.category_id = category.id',
           array())
    ->where('category_product.product_id=?', 123);

echo "\$select: ", $select, "\n";
// SELECT `category`.* FROM `category_product`
//  INNER JOIN `category` WHERE (category_product.product_id='123')
// WRONG:
//   1) Incorrect table in 'FROM'
//   2) Missing 'ON' clause in 'INNER JOIN'

// The workaround:
// This time, include a from()
$select = $categoryTable->select()
    ->from('category')
    ->join('category_product',
           'category_product.category_id = category.id',
           array())
    ->where('category_product.product_id=?', 123);

echo "\$select: ", $select, "\n";
// SELECT `category`.* FROM `category`
//  INNER JOIN `category_product` ON category_product.category_id=category.id
//  WHERE (category_product.product_id='123')
// *CORRECT*