Question

For example, a user create a product, and assign it into a category.

In the category, we have some record:

-phone, id:0
-computer, id:1
-pad, id:2

and a user create a product, the product need to assign a category, and a user do something like this:

name:iphone
cat:0

And the flow will work like this:

1.  check the cat is exist in db or not
2a. if not exist prompt error 
2b. if exist, put the record in db. 

But is there any chance when finished the step 1 checking, and before the 2b is start. The admin delete the phone category. If this happen, the database will insert a record which have no cat id watch. The db will have something like this:

*-phone, id:0* (not any more, because the admin delete it)
-computer, id:1
-pad, id:2

But there is a record like this:

name:iphone //but it belong to no category, because the admin delete it. 
cat:0

If there is a chance for doing this, how can I avoid this? Thank you.

(Using php and mysql with codeigniter)

Answer 1

There are two things you should think about:

1. Transactions – they make all changes atomic, i.e. all your changes are not visible for other users until you commit them, so everybody sees database in a consistent state.
2. Modify your database schema to use foreign keys – they ensure that data relations are kept, so if you set it up so that category must always exist and it suddenly disappears then your insert will fail.