Module: Positionable::PositionableMethods

Defined in:
lib/positionable.rb

Defined Under Namespace

Modules: InstanceMethods

Instance Method Summary collapse

Instance Method Details

#is_positionable(options = {}) ⇒ Object

Makes this model positionable.

class Item < ActiveRecord::Base
  is_positionable
end

Maybe your items are grouped (typically with a belongs_to association). In this case, you’ll want to restrict the position in each group by declaring the :scope option:

class Folder < ActiveRecord::Base
  has_many :items
end

class Item < ActiveRecord::Base
  belongs_to :folder
  is_positionable :scope => :folder
end

By default, position starts by zero. But you may want to change this at the model level, for instance by starting at one (which seems more natural for some people):

class Item < ActiveRecord::Base
  is_positionable :start => 1
end

To make new records to appear at first position, the default ordering can be changed as follows:

class Item < ActiveRecord::Base
  is_positionable :order => :desc
end



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# File 'lib/positionable.rb', line 62

def is_positionable(options = {})
  include InstanceMethods

  scope_id_attr = "#{options[:scope].to_s}_id" if options[:scope]
  start = options[:start] || 0
  order = options[:order] || :asc

  default_scope order("`#{self.table_name}`.`position` #{order}")

  before_create :add_to_bottom
  before_update :update_position
  after_destroy :decrement_all_next

  if scope_id_attr
    class_eval <<-RUBY

      def scope_id
        send(:"#{scope_id_attr}")
      end

      # Gives the range of available positions for this record, whithin the provided scope.
      # If no scope is provided, then it takes the record's current scope by default.
      # If this record is new and no scope can be retrieved, then a <tt>RangeWithoutScopeError</tt>
      # is raised.
      def range(scope = nil)
        raise RangeWithoutScopeError if new_record? and scope.nil? and scope_id.nil?
        # Does its best to retrieve the target scope...
        target_scope_id = scope.nil? ? scope_id : scope.id
        # Number of records whithin the target scope
        count = self.class.where("#{scope_id_attr} = ?", target_scope_id).count
        # An additional position is available if this record is new, or if it's moved to another scope
        if new_record? or target_scope_id != scope_id
          (start..(start + count))
        else
          (start..(start + count - 1))
        end
      end

    private

      def scoped_condition
        if scope_id.nil?
          "#{scope_id_attr} is null"
        else
          "#{scope_id_attr} = " + scope_id.to_s
        end
      end

      def scoped_position
        scoped_condition + " and position"
      end

      def scope_changed?
        send(:"#{scope_id_attr}_changed?")
      end

      def scope_id_was
        send(:"#{scope_id_attr}_was")
      end

      def scoped_condition_was
        "#{scope_id_attr} = " + scope_id_was.to_s
      end

      def scoped_position_was
        scoped_condition_was + " and position"
      end

    RUBY
  else
    class_eval <<-RUBY

      # Gives the range of available positions for this record.
      def range
        if new_record?
          (start..(bottom + 1))
        else
          (start..bottom)
        end
      end

    private

      def scope_changed?
        false
      end

      def scoped_condition
        ""
      end

      def scoped_position
        "position"
      end

    RUBY
  end

  class_eval <<-RUBY
    def start
      #{start}
    end
  RUBY
end