Best way to save a ordered List to the Database while keeping the ordering

Question

I was wondering if anyone has a good solution to a problem I\'ve encountered numerous times during the last years.

I have a shopping cart and my customer explicitly

Answer 1

On a level of abstraction above the cart Items let's say CartOrder (that has 1-n with CartItem) you can maintain a field called itemOrder which could be just a comma - separated list of id(PK) of cartItem records relevant . It will be at application layer that you require to parse that and arrange your item models accordingly . The big plus for this approach will be in case of order reshufflings , there might not be changes on individual objects but since order is persisted as an index field inside the order item table rows you will have to issue an update command for each one of the rows updating their index field. Please let me know your criticisms on this approach, i am curious to know in which ways this might fail.

Answer 2

Unfortunately there is no magic bullet for this. You cannot guarentee the order of any SELECT statement WITHOUT an order by clause. You need to add the column and program around it.

I don't know that I'd recommend adding gaps in the order sequence, depending on the size of your lists and the hits on the site, you might gain very little for the over head of handling the logic (you'd still need to cater for the occasion where all the gaps have been used up). I'd take a close look to see what benifits this would give you in your situation.

Sorry I can't offer anything better, Hope this helped.

Answer 3

FWIW, I think the way you suggest (i.e. committing the order to the database) is not a bad solution to your problem. I also think it's probably the safest/most reliable way.

Answer 4

How about using a linked list implementation? Having one column the will hold the value (order number) of the next item. I think it's by far the easiest to use when doing insertion of orders in between. No need to renumber.

Answer 5

Best solution is a Doubly Linked list. O(1) for all operations except indexing. Nothing can index SQL quickly though except a where clause on the item you want.

0,10,20 types fail. Sequence column ones fail. Float sequence column fails at group moves.

Doubly Linked list is same operations for addition, removal, group deletion, group addition, group move. Single linked list works ok too. Double linked is better with SQL in my opinion though. Single linked list requires you to have the entire list.

Answer 6

I wouldn't recommend the A, AA, B, BA, BB approach at all. There's a lot of extra processing involved to determine hierarchy and inserting entries in between is not fun at all.

Just add an OrderField, integer. Don't use gaps, because then you have to either work with a non-standard 'step' on your next middle insert, or you will have to resynchronize your list first, then add a new entry.

Having 0...N is easy to reorder, and if you can use Array methods or List methods outside of SQL to re-order the collection as a whole, then update each entry, or you can figure out where you are inserting into, and +1 or -1 each entry after or before it accordingly.

Once you have a little library written for it, it'll be a piece of cake.