sorting.lab.rb

# frozen_string_literal: true

require 'sqlite3'
require 'active_record'
require 'ap'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
  create_table :todos do |t|
    t.string :label
    t.integer :position
  end
end

ActiveRecord::Base.logger = Logger.new(STDOUT)

class ApplicationRecord < ActiveRecord::Base

  self.abstract_class = true

end

class Todo < ApplicationRecord; end

Todo.create! label: 'Green', position: 0
Todo.create! label: 'Blue', position: 1
Todo.create! label: 'Red', position: 2

todos = Todo.order(:position).to_a

todos_new_order = todos.rotate(-1)

todos_new_order.each_with_index { |todo, i| todo.position = i }

todos_new_order.map(&:save)

# Result: Red, Green, Blue