ActiveRecord Postgres Nulls Last

nulls_last.rb

ActiveSupport.on_load(:active_record) do
  module Arel::NullsLastPredications
    def nulls_last
      Arel::Nodes::NullsLast.new self
    end
  end

  module Arel::Nodes
    class NullsLast < Unary
      def gsub *args
        expr.to_sql.gsub *args
      end
    end
    Ordering.send(:include, Arel::NullsLastPredications)
  end

  module Arel::Visitors
    class DepthFirst
      private

      alias :visit_Arel_Nodes_NullsLast :unary
    end

    class PostgreSQL
      private

      def visit_Arel_Nodes_NullsLast o
        "#{visit o.expr} NULLS LAST"
      end
    end
  end
end

#Sample use:
users = User.arel_table
User.order(users[:first_name].desc.nulls_last)

If someone want the above snippet to work in Rails > 4 and Arel v. 6 change the visit_Arel_Nodes_NullsLast method in PostgreSQL class to:

      def visit_Arel_Nodes_NullsLast o, collector
        (visit o.expr, collector) << ' NULLS LAST'
      end