diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ea415ee
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.phan/
diff --git a/index.php b/index.php
index 5870efe..63cc209 100644
--- a/index.php
+++ b/index.php
@@ -1,5 +1,7 @@
 <?php
 require 'autoload.php';
+ini_set('display_errors', true);
+ini_set('error_log', '/tmp/php.log');
 
 use ProductList\Http\Request;
 use ProductList\Http\RequestHandler;
diff --git a/src/Exception/DuplicateException.php b/src/Exception/DuplicateException.php
new file mode 100644
index 0000000..b0359da
--- /dev/null
+++ b/src/Exception/DuplicateException.php
@@ -0,0 +1,4 @@
+<?php
+namespace ProductList\Exception;
+
+class DuplicateException extends \Exception {}
diff --git a/src/Exception/InvalidFieldException.php b/src/Exception/InvalidFieldException.php
new file mode 100644
index 0000000..63be3f4
--- /dev/null
+++ b/src/Exception/InvalidFieldException.php
@@ -0,0 +1,11 @@
+<?php
+namespace ProductList\Exception;
+
+class InvalidFieldException extends \Exception
+{
+	public function __construct($field, $code = 0, Throwable $previous = null)
+	{
+		http_response_code(400);
+		parent::__construct("The field '$field' is invalid.", $code, $previous);
+	}
+}
diff --git a/src/Model/Book.php b/src/Model/Book.php
index a62c8cf..b48b62c 100644
--- a/src/Model/Book.php
+++ b/src/Model/Book.php
@@ -1,6 +1,8 @@
 <?php
 namespace ProductList\Model;
 
+use ProductList\Exception\InvalidFieldException;
+
 class Book extends Product
 {
     private $weight;
@@ -14,6 +16,11 @@ class Book extends Product
         $variationId = null
     ) {
         parent::__construct($sku, $name, $price, $productId, $variationId);
+
+        if (empty($weight) || !is_numeric($weight)) {
+            throw new InvalidFieldException('Weight');
+        }
+
         $this->weight = $weight;
     }
 
@@ -24,8 +31,8 @@ class Book extends Product
             $row['name'],
             $row['price'],
             $row['weight'],
-            $row['product_id'],
-            $row['variation_id']
+            $row['productId'],
+            $row['variationId']
         );
     }
 
@@ -42,8 +49,15 @@ class Book extends Product
 
     private static function getSelectAllQuery() : string
     {
-        return 'SELECT '.PRODUCT.'.*, '.BOOK.'.id as variation_id, size
-            FROM '.PRODUCT.' LEFT JOIN '.BOOK.' ON '.PRODUCT.'.id = '.BOOK.'.product_id';
+        return '
+        SELECT
+                '.PRODUCT.'.*,
+                '.BOOK.'.id as variationId,
+                weight
+        FROM
+                '.PRODUCT.'
+        LEFT JOIN
+                '.BOOK.' ON '.PRODUCT.'.id = '.BOOK.'.product_id';
     }
 
     public function insert($conn = null) : int
@@ -81,7 +95,7 @@ class Book extends Product
         $stmt->bind_param('i', $variationId);
 
         if ($stmt->execute() === false) {
-            throw new \Exception("Unable to delete product with id '$id'");
+            throw new \Exception("Unable to delete product with id '$variationId'");
         }
 
         parent::delete($conn);
diff --git a/src/Model/DVD.php b/src/Model/DVD.php
index 6e05e39..a8cb60c 100644
--- a/src/Model/DVD.php
+++ b/src/Model/DVD.php
@@ -1,6 +1,8 @@
 <?php
 namespace ProductList\Model;
 
+use ProductList\Exception\InvalidFieldException;
+
 class DVD extends Product
 {
     private $size;
@@ -14,6 +16,11 @@ class DVD extends Product
         $variationId = null
     ) {
         parent::__construct($sku, $name, $price, $productId, $variationId);
+
+        if (empty($size) || !is_numeric($size)) {
+            throw new InvalidFieldException('Size');
+        }
+
         $this->size = $size;
     }
 
@@ -24,8 +31,8 @@ class DVD extends Product
             $row['name'],
             $row['price'],
             $row['size'],
-            $row['product_id'],
-            $row['variation_id']
+            $row['productId'],
+            $row['variationId']
         );
     }
 
@@ -42,8 +49,15 @@ class DVD extends Product
 
     private static function getSelectAllQuery() : string
     {
-        return 'SELECT '.PRODUCT.'.*, '.DVD.'.id as variation_id, size
-            FROM '.PRODUCT.' LEFT JOIN '.DVD.' ON '.PRODUCT.'.id = '.DVD.'.product_id';
+        return '
+        SELECT
+                '.PRODUCT.'.*,
+                '.DVD.'.id as variationId,
+                size
+        FROM
+                '.PRODUCT.'
+        LEFT JOIN
+                '.DVD.' ON '.PRODUCT.'.id = '.DVD.'.product_id';
     }
 
     public function insert($conn = null) : int
@@ -66,7 +80,7 @@ class DVD extends Product
             $this->setVariationId($conn->insert_id);
             return $conn->insert_id;
         } else {
-            throw new Exception("Unable to insert object");
+            throw new \Exception("Unable to insert object");
         }
     }
 
@@ -81,7 +95,7 @@ class DVD extends Product
         $stmt->bind_param('i', $variationId);
 
         if ($stmt->execute() === false) {
-            throw new \Exception("Unable to delete product with id '$id'");
+            throw new \Exception("Unable to delete product with id '$variationId'");
         }
 
         parent::delete($conn);
diff --git a/src/Model/Furniture.php b/src/Model/Furniture.php
index 91d558d..51902b9 100644
--- a/src/Model/Furniture.php
+++ b/src/Model/Furniture.php
@@ -1,6 +1,8 @@
 <?php
 namespace ProductList\Model;
 
+use ProductList\Exception\InvalidFieldException;
+
 class Furniture extends Product
 {
     private $height;
@@ -8,7 +10,7 @@ class Furniture extends Product
     private $length;
 
     public function __construct(
-        $SKU,
+        $sku,
         $name,
         $price,
         $height,
@@ -17,7 +19,20 @@ class Furniture extends Product
         $productId = null,
         $variationId = null
     ) {
-        parent::__construct($SKU, $name, $price, $productId, $variationId);
+        parent::__construct($sku, $name, $price, $productId, $variationId);
+
+        if (empty($height) || !is_numeric($height)) {
+            throw new InvalidFieldException('Height');
+        }
+
+        if (empty($width) || !is_numeric($width)) {
+            throw new InvalidFieldException('Width');
+        }
+
+        if (empty($length) || !is_numeric($length)) {
+            throw new InvalidFieldException('Length');
+        }
+
         $this->height = $height;
         $this->width = $width;
         $this->length = $length;
@@ -32,8 +47,8 @@ class Furniture extends Product
             $row['height'],
             $row['width'],
             $row['length'],
-            $row['product_id'],
-            $row['variation_id']
+            $row['productId'],
+            $row['variationId']
         );
     }
 
@@ -62,9 +77,17 @@ class Furniture extends Product
 
     private static function getSelectAllQuery() : string
     {
-        return 'SELECT '.PRODUCT.'.*, '.FURNITURE.'.id as variation_id, size
-            FROM '.PRODUCT.'
-            LEFT JOIN '.FURNITURE.' ON '.PRODUCT.'.id = '.FURNITURE.'.product_id';
+        return '
+        SELECT
+                '.PRODUCT.'.*,
+                '.FURNITURE.'.id as variationId,
+                height,
+                width,
+                length
+        FROM
+                '.PRODUCT.'
+        LEFT JOIN '.FURNITURE.' ON
+                '.PRODUCT.'.id = '.FURNITURE.'.product_id';
     }
 
     public function insert($conn = null) : int
@@ -107,7 +130,7 @@ class Furniture extends Product
         $stmt->bind_param('i', $variationId);
 
         if ($stmt->execute() === false) {
-            throw new \Exception("Unable to delete product with id '$id'");
+            throw new \Exception("Unable to delete product with id '$variationId'");
         }
 
         parent::delete($conn);
diff --git a/src/Model/Product.php b/src/Model/Product.php
index bf9f427..ba05bc6 100644
--- a/src/Model/Product.php
+++ b/src/Model/Product.php
@@ -2,33 +2,49 @@
 namespace ProductList\Model;
 
 use ProductList\Exception\NotFoundException;
+use ProductList\Exception\InvalidFieldException;
+use ProductList\Exception\DuplicateException;
 
 abstract class Product implements \JsonSerializable
 {
     use Model;
     private $variationId;
-    private $SKU;
+    private $sku;
     private $name;
     private $price;
     private $productId;
 
     public function __construct(
-        $SKU,
+        $sku,
         $name,
         $price,
         $productId = null,
         $variationId = null
     ) {
+        if (empty($sku)) {
+            throw new InvalidFieldException('SKU');
+        }
+
+        if (empty($name)) {
+            throw new InvalidFieldException('Name');
+        }
+
+        if (empty($price) || !is_numeric($price)) {
+            throw new InvalidFieldException('Price');
+        }
+
         $this->productId = $productId;
         $this->variationId = $variationId;
-        $this->SKU = $SKU;
+
+
+        $this->sku = $sku;
         $this->name = $name;
         $this->price = $price;
     }
 
     public function getSKU()
     {
-        return $this->SKU;
+        return $this->sku;
     }
 
     public function getName()
@@ -74,7 +90,7 @@ abstract class Product implements \JsonSerializable
         $stmt->bind_param('i', $productId);
 
         if ($stmt->execute() === false) {
-            throw new \Exception("Unable to delete product with id '$id'");
+            throw new \Exception("Unable to delete product with id '$productId'");
         }
     }
 
@@ -102,26 +118,45 @@ abstract class Product implements \JsonSerializable
 
     public static function fromRow($row) : self
     {
-        if ($row['size'] !== null) {
-            return DVD::fromRow($row);
-        } elseif ($row['weight'] !== null) {
-            return Book::fromRow($row);
-        } elseif ($row['height'] !== null) {
-            return Furniture::fromRow($row);
-        } else {
-            throw new \Exception("Product without a type");
+        switch ($row['productType']) {
+            case 'dvd':
+                return DVD::fromRow($row);
+            case 'book':
+                return Book::fromRow($row);
+            case 'furniture':
+                return Furniture::fromRow($row);
+            default:
+                throw new \Exception("Product without a type");
         }
     }
 
     private static function getSelectAllQuery() : string
     {
-        return 'SELECT '.PRODUCT.'.id as product_id,
-                    COALESCE('.DVD.'.id, '.BOOK.'.id, '.FURNITURE.'.id) as variation_id,
-                    name, sku, price, size, weight, width, height, length
-                FROM '.PRODUCT.'
-                LEFT JOIN '.DVD.' ON '.PRODUCT.'.id = '.DVD.'.product_id
-                LEFT JOIN '.BOOK.' ON '.PRODUCT.'.id = '.BOOK.'.product_id
-                LEFT JOIN '.FURNITURE.' ON '.PRODUCT.'.id = '.FURNITURE.'.product_id';
+        return '
+        SELECT
+                '.PRODUCT.'.id as productId,
+                name,
+                sku,
+                price,
+                size,
+                weight,
+                width,
+                height,
+                length,
+                COALESCE('.DVD.'.id, '.BOOK.'.id, '.FURNITURE.'.id) as variationId,
+                CASE
+                        WHEN '.DVD.'.id IS NOT NULL THEN "dvd"
+                        WHEN '.BOOK.'.id IS NOT NULL THEN "book"
+                        WHEN '.FURNITURE.'.id IS NOT NULL THEN "furniture"
+                END as productType
+        FROM
+                '.PRODUCT.'
+        LEFT JOIN '.DVD.' ON
+                '.PRODUCT.'.id = '.DVD.'.product_id
+        LEFT JOIN '.BOOK.' ON
+                '.PRODUCT.'.id = '.BOOK.'.product_id
+        LEFT JOIN '.FURNITURE.' ON
+                '.PRODUCT.'.id = '.FURNITURE.'.product_id';
     }
 
     public function insert($conn = null) : int
@@ -140,11 +175,19 @@ abstract class Product implements \JsonSerializable
         );
         $stmt->bind_param('ssd', $SKU, $name, $price);
 
-        if ($stmt->execute() === true) {
-            $this->setProductId($conn->insert_id);
-            return $conn->insert_id;
-        } else {
-            throw new \Exception("Unable to insert object");
+        try {
+            if ($stmt->execute() === true) {
+                $this->setProductId($conn->insert_id);
+                return $conn->insert_id;
+            } else {
+                throw new \Exception("Unable to insert object");
+            }
+        } catch (\mysqli_sql_exception $e) {
+            if ($e->getCode() === 1062) {
+                throw new DuplicateException('The provided SKU is already registered.');
+            } else {
+                throw $e;
+            }
         }
     }
 
diff --git a/src/View/Product.php b/src/View/Product.php
index ed1b230..0122f74 100644
--- a/src/View/Product.php
+++ b/src/View/Product.php
@@ -3,9 +3,6 @@ namespace ProductList\View;
 
 use ProductList\Http\Request;
 use ProductList\Model\Product as ProductModel;
-use ProductList\Model\DVD;
-use ProductList\Model\Furniture;
-use ProductList\Model\Book;
 use ProductList\Exception\NotFoundException;
 
 class Product extends View
@@ -38,7 +35,7 @@ class Product extends View
 
                 } catch (NotFoundException $e) {
                     http_response_code(404);
-                    echo $e->getMessage();
+                    echo "The selected(s) object(s) is(are) not available anymore.";
                     return;
                 }
             }
@@ -63,55 +60,12 @@ class Product extends View
             'length'
         ];
         if (self::expectArgs($expected, $params)) {
-            $product = null;
-            $type = $params['productType'];
+            $params['productId'] = null;
+            $params['variationId'] = null;
 
-            switch($type) {
-                case 'dvd':
-                    $product = new DVD(
-                        $params['sku'],
-                        $params['name'],
-                        $params['price'],
-                        $params['size'],
-                    );
-                    break;
-                case 'furniture':
-                    $product = new Furniture(
-                        $params['sku'],
-                        $params['name'],
-                        $params['price'],
-                        $params['height'],
-                        $params['width'],
-                        $params['length'],
-                    );
-                    break;
-                case 'book':
-                    $product = new Book(
-                        $params['sku'],
-                        $params['name'],
-                        $params['price'],
-                        $params['weight'],
-                    );
-                    break;
-                default:
-                    http_response_code(400);
-                    echo "Invalid 'productType' value '$type'";
-                    return;
-            }
+            $product = ProductModel::fromRow($params);
 
-            //try {
-                $product->insert();
-            //} catch (\Exception $e) {
-
-            //}
-        }
-    }
-
-    public static function test(Request $request)
-    {
-        $params= $request->getQueryParams();
-        if (self::expectArgs(['testarg'], $params)) {
-            echo var_dump($params);
+            $product->insert();
         }
     }
 }
diff --git a/static/index.js b/static/index.js
index 70b7db8..faa1eb0 100644
--- a/static/index.js
+++ b/static/index.js
@@ -29,12 +29,16 @@ $('#delete-product-btn').on('click', () => {
     const checkboxes = document.querySelectorAll('input[class="delete-checkbox"]:checked');
     checkboxes.forEach(checkbox => values.push(checkbox.value));
 
-    $.ajax(
-        `product?id=${values.join(',')}`,
-        {
-            method: 'DELETE',
-            success: loadItems,
-            error: jqXHR => alert(jqXHR.responseText),
-        }
-    )
+    if(values.length) {
+        $.ajax(
+            `product?id=${values.join(',')}`,
+            {
+                method: 'DELETE',
+                success: loadItems,
+                error: jqXHR => alert(jqXHR.responseText),
+            }
+        )
+    } else {
+        alert('Please select a product.');
+    }
 });