arel.rb

module Arel
  
  def self.add_predicate(predicate, operator)
    
    $method_name = predicate
    $operator = operator
    $class_name = $method_name.to_s.camelcase
    
    module Arel::Predications
      define_method($method_name) do |right|
        :"Arel::Nodes::#{$class_name}".to_s.constantize.new(self, quoted_node(right))
      end
    end

    node = Class.new(Arel::Nodes::Binary) do
      def operator; $operator end
    end
    Arel::Nodes.const_set($class_name, node)

    class Arel::Visitors::PostgreSQL
      private
      define_method(:"visit_Arel_Nodes_#{$class_name}") do |o, collector|
        infix_value o, collector, " #{$operator} "
      end
    end
    
  end
  
end