A sequel plugin that allows positioning with options.

positioning.rb

module Sequel
  module Plugins
    module Positioning
      def self.apply(model, options = {})
        options[:field] ||= :position

        field = options[:field]
        scope = options[:scope]

        if scope
          model.dataset.order!(scope, field)
        else
          model.dataset.order!(field)
        end

        model.instance_eval <<-CODE
          def positioning_options
            #{options.inspect}
          end
        CODE
      end

      module InstanceMethods
        def move_to(p, s = scope_value)
          new_position        = p.to_i
          new_scope_value     = s && s.to_i
          current_position    = position_value
          current_scope_value = scope_value

          raise "Moving too far up"   if new_position < 0
          raise "Moving too far down" if new_position > last_position_value

          db.transaction do
            if new_scope_value == current_scope_value 

              case
              when new_position < current_position
                ds = self.class.dataset.filter("? >= ? AND ? < ?", position_field, new_position, position_field, current_position)
                ds = ds.filter(scope_field => scope_value) if scope_field
                ds.update(position_field => %Q{"#{position_field}" + 1}.lit)
              when new_position > current_position
                ds = self.class.dataset.filter("? > ? AND ? <= ?", position_field, current_position, position_field, new_position)
                ds = ds.filter(scope_field => scope_value) if scope_field
                ds.update(position_field => %Q{"#{position_field}" - 1}.lit)
              else
                return self
              end

              update(position_field => new_position)

            else

              ds = self.class.dataset.filter("? > ? AND ? = ?", position_field, current_position, scope_field, current_scope_value)
              ds.update(position_field => %Q{"#{position_field}" - 1}.lit)

              ds = self.class.dataset.filter("? >= ? AND ? = ?", position_field, new_position, scope_field, new_scope_value)
              ds.update(position_field => %Q{"#{position_field}" + 1}.lit)

              update(position_field => new_position, scope_field => new_scope_value)

            end
          end
        end

        def before_create
          super
          @values[position_field] = (last_position_value ? (last_position_value + 1) : 0)
        end

        def after_destroy 
          super
          ds = self.class.dataset.filter("? > ?", position_field, position_value)
          ds = ds.filter(scope_field => scope_value) if scope_field
          ds.update(position_field => %Q{"#{position_field}" - 1}.lit)
        end

        private

        def position_field
          self.class.positioning_options[:field]
        end

        def position_value
          @values[position_field]
        end

        def scope_field
          self.class.positioning_options[:scope]
        end

        def scope_value
          scope_field && @values[scope_field]
        end

        def last_position_value
          ds = self.class.dataset
          ds = ds.filter(scope_field => scope_value) if scope_field
          ds.max(position_field)
        end
      end
    end
  end
end