I have to agree 100%. It's easy with an application like Rails for beginning programmers to wreak havoc on a server by not being careful with their database queries. Unfortunately, this tutorial does just that in the "Showing recipes in a category" code.
The working 'list' method of the Recipe controller should look like this:
@category = @params['category']
As pointed out by petercooper, this allows the database server to weed out all records that do not match, rather than loading them all into memory so that the view can filter them.