active_data_table.rb

class ActiveDataTables

    def initialize(query, params)
      @query = query
      @params = params
    end

    def execute
      total = @query.count
      filtered = total # don't currently support filtering records so this will be the same as the total

      apply_paging
      apply_ordering

      DataTablesResult.new(@params[:draw].to_i, @query, total, filtered)
    end

    def self.find(query, params)
      new(query, params).execute
    end

    class DataTablesResult

      attr_accessor :data, :draw, :records_total, :records_filtered

      def initialize(draw, data, records_total, records_filtered)
        @draw = draw
        @data = data
        @records_total = records_total
        @records_filtered = records_filtered
      end

      def to_json
        Jbuilder.encode do |json|
          json.draw @draw
          json.recordsTotal @records_total
          json.recordsFiltered @records_filtered
          json.data @data
        end
      end

    end

    private

    def apply_paging
      @query = @query.offset(@params[:start]).limit(@params[:length])
    end

    def apply_ordering
      order_instructions = @params[:order] || {}

      columns = load_columns
      order = {}

      order_instructions.each_pair do |k, v|
        column_index = v[:column]
        column_name = columns[column_index.to_sym][:data]
        order[column_name.to_sym] = v[:dir].to_sym
      end

      @query = @query.reorder(order)
    end

    def load_columns
      @params[:columns] || {}
    end

  end