Question

I would like to allow users to specify the sequence of items in the same way that I can sort my Netflix Queue. For example, given a view:

 #  |  Item Name
----+------------
 1  |  one
 2  |  three
 3  |  four
 4  |  two

If the user changes the order for Item Name “two” to “2” and saves, then they should see:

 #  |  Item Name
----+------------
 1  |  one
 2  |  two
 3  |  three
 4  |  four

I’ve tried using a before_save:

class List < ActiveRecord::Base
  has_many :items

  before_save :sequence_items

  def sequence_items
    items.sort{|f,s|f.number <=> s.number}.each_with_index do |item, index|
      item.update_attributes(:number => index + 1)
    end
  end
end

but this results in:

 #  |  Item Name
----+------------
 1  |  one
 2  |  three
 3  |  two
 4  |  four

Answer 1

I found that I can use the acts_as_list plugin. I renamed the number attribute in the Item table to position, then modified code as below:

class Item < ActiveRecord::Base
  belongs_to :list

  acts_as_list
end

class List < ActiveRecord::Base
  has_many :items

  before_save :sequence_items

  def sequence_items
    items.each_with_index do |item, index|
      item.insert_at(item.position || index + 1)
    end
  end
end

Now whenever the list is saved each item will be inserted at the provided position. I have many existing items that do not have a position value so I added the || index + 1 to ensure that they get assigned a position at the end of the list.