Haversine formula with Arel, with possibility of portability.

address.rb

class Address < ActiveRecord::Base

  def self.order_proximity_to(latitude, longitude)
    order(Arel::Nodes::Haversine.new(arel_table[:latitude],
                                     arel_table[:longitude],
                                     latitude,
                                     longitude))
  end

end

arel_extensions.rb

ActiveSupport.on_load(:active_record) do
  module Arel::Nodes
    class Arcsine < Unary; end
    class Cosine < Unary; end
    class Radians < Unary; end
    class Sine < Unary; end
    class Sqrt < Unary; end
    class Pow < InfixOperation
      def initialize left, right
        super(:^, left, right)
      end
    end
    class Haversine < Unary
      def initialize lat1, lng1, lat2, lng2
        @expr = Multiplication.new(
          Arcsine.new(
            Sqrt.new(
              Addition.new(
                Pow.new(Sine.new(Division.new(Radians.new(lat1 - lat2), 2)), 2),
                Multiplication.new(
                  Pow.new(Sine.new(Division.new(Radians.new(lng1 - lng2), 2)), 2),
                  Multiplication.new(
                    Cosine.new(Radians.new(lat2)),
                    Cosine.new(Radians.new(lat1))
                  )
                )
              )
            )
          ), 12742
        )
      end
    end
  end

  class Arel::Visitors::ToSql
    private

    def visit_Arel_Nodes_Haversine o
      visit o.expr
    end
  end

  class Arel::Visitors::PostgreSQL
    private

    def visit_Arel_Nodes_Arcsine o
      "ASIN(#{visit o.expr})"
    end

    def visit_Arel_Nodes_Cosine o
      "COS(#{visit o.expr})"
    end

    def visit_Arel_Nodes_Radians o
      "RADIANS(#{visit o.expr})"
    end

    def visit_Arel_Nodes_Sine o
      "SIN(#{visit o.expr})"
    end

    def visit_Arel_Nodes_Sqrt o
      "SQRT(#{visit o.expr})"
    end
  end
end