Adds .previous and .next named scopes to your Active Record model, treating the records in your database like a sequential list with an order of your choosing.

acts_as_sequential.rb

# Adds .previous and .next class methods to your model treating the records in your database like a sequential list
#
# Put this in your model definition:
#     acts_as_sequential :column => :created_at # you can also use the :dir => 'DESC' option (ASC is the default)
#
# Then use it like this:
#     Model.next(current_record) -> next record in sequence
#     Model.other_named_scope.previous(current_record) -> previous record in sequence
#

module ActsAsSequential
	class << self
		def included(base)
			base.extend ClassMethods
		end
	end

	module ClassMethods
		def acts_as_sequential opt={}
			column = opt[:column]

			table_name = self.table_name
			directions = [:next, :previous]
			
			desc_condition = "#{table_name}.#{column} <= ? AND #{table_name}.id != ?"
			asc_condition  = "#{table_name}.#{column} >= ? AND #{table_name}.id != ?"
			
			directions.reverse! if opt[:dir].to_s =~ /desc/i

			class_eval %Q?
				def self.#{directions.pop}(record)
					self.first(
						:order => "#{table_name}.#{column} DESC",
						:conditions => ["#{desc_condition}", record["#{column}"], record.id]
					)
				end
			?
			
			class_eval %Q?
				def self.#{directions.pop}(record)
					self.first(
						:order => "#{table_name}.#{column} ASC",
						:conditions => ["#{asc_condition}", record["#{column}"], record.id]
					)
				end
			?
		end
	end
end

if Object.const_defined?("ActiveRecord")
	ActiveRecord::Base.send(:include, ActsAsSequential)
end