Automatically enter serial value for sort field based on Product_ID

[ah]

Hi, I am trying to find a simple solution to solve the following problem:

I have a table that stores images for a product. Thus one product can have many images.

To decide how they will be shown, these images need to be sorted, and have a unique sort value.

Is there a simple way to find the highest number in the sort field where the product_ID is the same, and auto increment this?

Also, does anyone have a good solution for changing these values in a simple way?

Kind of the way tab order is set up in filemaker, so that if you change the value to a lower one, all the other images with the same product_ID will automatically increment their values with one.

Thanks in advance

[comment]

See if this helps:

http://fmforums.com/forum/topic/60325-manually-reorder-portal-records/#entry285451

[ah]

That certainly helped a lot, thanks :)