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copied!<p>I would suggest subtracting the sold items right away from <code>Product::available_items</code>. This would resolve the issue right away, because <code>Product::available_items</code> would always reflect the number of available items.</p> <p><strong>Method no. 1</strong></p> <p>Subtract <code>Sale::quantity</code> from <code>Product</code> right away.</p> <pre class="lang-php prettyprint-override"><code>public function setProduct(ProductInterface $product) { $new_count = $product->getAvailableItems() - $this->getQuantity(); $product->setAvailableItems($new_count); $this->product = $product; } </code></pre> <p>You'd also have to do the subtracting in all the controllers that might change the available item count.</p> <hr> <p><strong>Method no. 2</strong></p> <p>Personally, I would actually favor this method over the first one, because it keeps the logic in a single place and generally results in shorter controllers. In addition, my entities are usually long enough without all the domain logic.</p> <p>Assuming <code>Sale</code> has a reference to a <code>Product</code>, I'd put the logic into a service (an entity-specific entitymanager, e.g <a href="https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Model/UserManagerInterface.php" rel="nofollow"><code>UserManager</code></a>. It'd be called <code>SaleManager</code> in our case). The method in it would look something similar to:</p> <pre class="lang-php prettyprint-override"><code>public function updateSale(SaleInterface $sale, $flush = true) { $product = $sale->getProduct(); $new_count = $product->getAvailableItems() - $sale->getQuantity(); if ($new_count < 0) { throw new NegativeQuantityException(); } $product->setAvailableItems($new_count); $this->em->persist($sale); if (true === $flush) { $this->em->flush(); } } </code></pre> <p>I didn't touch the issue of what would happen if you were to remove a <code>Sale</code> from a <code>Product</code> or switch a <code>Product in a sale</code>, but you must certainly consider this as well.</p>

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