book.rb

class Book < ActiveRecord::Base
  scope :category, lambda {|cat| where(:category_id => cat) }
  scope :author, lambda {|auth| where(:authori_id => auth) }
  
  # example of a method to build chainable scope
  # based on the query parameters
  def self.query params={}
    scoped = limit(25) 

    scoped = scoped.category( params[:category_id] ) if params[:category_id].present?

    scoped = scoped.author( params[:author_id] ) if params[:author_id].present?

    scoped
  end
end

class BooksController < ApplicationController
  def index
    @books = Book.query(params)

    if request.xhr?
      # renders just a list of records
      # for use by jquery
      render :partial => "books/listing"
    else
      # renders full template, with layout
      render 
    end
  end
end