Question

The table ps_category_product in PrestaShop has the following structure

 # Obtained using SHOW CREATE TABLE `ps_category_product`
 CREATE TABLE `ps_category_product` (
   `id_category` int(10) unsigned NOT NULL,
   `id_product` int(10) unsigned NOT NULL,
   `position` int(10) unsigned NOT NULL DEFAULT '0',
   KEY `category_product_index` (`id_category`,`id_product`),
   KEY `id_product` (`id_product`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

For me is not very clear, but it seems that the fields id_category and id_product should be unique among the table, but for some reason MySQL allows me to insert duplicates:

mysql> select * from ps_category_product limit 10;
+-------------+------------+----------+
| id_category | id_product | position |
+-------------+------------+----------+
|          11 |          1 |        1 |
|          11 |          2 |        1 |
|          11 |          3 |        1 |
|          11 |          4 |        1 |
|          11 |          5 |        1 |
|          11 |          6 |        1 |
|          11 |          7 |        1 |
|          11 |          8 |        1 |
|          11 |          9 |        1 |
|          11 |         10 |        1 |
+-------------+------------+----------+
10 rows in set (0.00 sec)

mysql> INSERT INTO `ps_category_product` VALUES(11, 1, 1);
Query OK, 1 row affected (0.05 sec)

How can I prevent this from happening?

Later edit

It was a bug in prestashop. Take a look at http://forge.prestashop.com/browse/PSCFI-4397

Answer 1

Specifying KEY will not enforce a unique constraint unless you specify UNIQUE KEY or PRIMARY KEY.

Try recreating the table using the following DDL:

CREATE TABLE `ps_category_product` (
   `id_category` int(10) unsigned NOT NULL,
   `id_product` int(10) unsigned NOT NULL,
   `position` int(10) unsigned NOT NULL DEFAULT '0',
   UNIQUE KEY `category_product_index` (`id_category`,`id_product`),
   KEY `id_product` (`id_product`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

That should do the trick.

Have a look at the MySQL CREATE TABLE syntax for more info.