fix cartItem updateQuantity method

core/lib/Thelia/Model/CartItem.php

@@ -60,7 +60,14 @@ class CartItem extends BaseCartItem
             }
         }
 
-        $this->setQuantity($value);
+        $this->addQuantity($value);
+
+        return $this;
+    }
+
+    public function addQuantity($quantity)
+    {
+        $this->setQuantity($this->getQuantity() + $quantity);
 
         return $this;
     }

templates/default/product.html

@@ -138,6 +138,10 @@
                     {form_hidden_fields form=$form}
                     <input type="hidden" name="view" value="product">
                     <input type="hidden" name="product_id" value="{$ID}">
+                    {form_field form=$form field="append"}
+                        <input type="hidden" name="{$name}" value="1">
+                    {/form_field}
+
 
                     {if $form_error}<div class="alert alert-error">{$form_error_message}</div>{/if}