added some features to sortable_columns

[aldavidson]

Hi,

Thanks for sortable_columns, it's really handy. If you're interested, I've added a couple of features in my fork:

1. ability to sort by a model's instance methods, as well as attributes

This is really useful when you're sorting a collection of models by some attribute on an associated record (e.g. to sort @payments by payment.account.name, you'd define :

class Payment < ActiveRecord::Base
belongs_to :account

sortable_methods [:account_name, :some_other_method]

def account_name
self.account.name
end
end

( I added the sortable_methods requirement to avoid a security hole - e.g. sort_by=destroy)

2. when you're on a url with non-sort parameters (e.g. /things/index?parent_id=1234 ) it now maintains existing params when you click on a sortable column header

3. As a result of the other two changes, I centralized the logic for validating sort params into one place rather than two

Cheers,

Al