filter.rb

class Filter
  attr_reader :rows

  def initialize(filtered_class)
    @filtered_class = filtered_class
    @rows = []
  end

  def add_row(row)
    @rows << row
  end

  def results
    @filtered_class.where(row_to_arel(@rows[0]))
  end

  private

  def table
    @table ||= Arel::Table.new(@filtered_class.table_name)
  end

  def row_to_arel(row = {})
    return if row.nil?

    columns = row.collect do |k, v|
      column = k
      operator, value = v.flatten
      table[column].send(operator, value)
    end

    columns.reduce(:and)
  end

end