datatable with arel and reflection

gistfile1.rb

# 1. data table subclass is initialized
#   class methods are going to be called on the class instance, and the class will store table data
#
# 2. instantiate an instance of the data table subclass ( OrdersIndex.new)
#
# 3. query the instance w/ pagination and sorting params
#    a query gets executed w/ params -> ARel
#    results get stored  -> AR
#    results get passed back as json
class DataTable

  def self.current_klass
    @inner_klass || @klass
  end

  def self.relation
    @relation
  end

  #
  # always passing an arel object in select ensures
  # that we always get fully qualified sql queries
  # using the 'table'.'column' format
  #
  def self.column(name)
    @relation = relation.select(current_klass.arel_table[name])
  end

  #
  # keep the inner_klass the actual class keeps it more consistent
  # with the klass variable. Using reflect_on_association through
  # the class should ensure we don't have to worry about guessing
  # table names from model names.
  #
  def self.join(association, &block)
    @inner_klass = current_klass.reflect_on_association(association).klass
    @relation = relation.joins(association)
    instance_eval(&block) if block_given?
    @inner_klass = nil
  end

  def self.set_model(klass)
    @klass = klass
    @relation = klass
  end

  attr_accessor :echo
  attr_accessor :data
  attr_accessor :total_count
  attr_accessor :displayed_count

  def initialize(params={})
    @echo = (params['sEcho'] || -1).to_i
    @data = []
    @displayed_count = 0
    @total_count = 0
  end

  def json
    {
      'sEcho' => echo,
      'aaData' => data,
      'iTotalRecords' => total_count,
      'iTotalDisplayRecords' => displayed_count,
      'aaRecords' => []
    }
  end

  def query(params)
    self
  end

  def sql
    self.class.relation.to_sql
  end


end