Further to runako's explanation, it's actually pretty useful to have the choice of whether an exception is raised or not. I'm working on a blog application and I wanted to add support for viewing the next or previous blog entry. I was able to add two instance methods to my Post model that simply return nil when you try to get the previous post when viewing the first post, or the next post when viewing the last post:
def next
Post.find_by_id(id + 1)
end
def previous
Post.find_by_id(id - 1)
end
This avoids my helper code which conditionally generates the Previous Post/Next Post links from having to handle the RecordNotFound exception, which would be bad because it would be using an exception for control flow.
