kanban_placements.rb

require "bundler/inline"

gemfile do
  gem "activerecord", require: "active_record"
  gem "pg"
end

ActiveRecord::Base.logger = Logger.new STDOUT

begin
 ActiveRecord::Tasks::DatabaseTasks.drop(adapter: "postgresql", database: "kanban_boards")
 ActiveRecord::Tasks::DatabaseTasks.create(adapter: "postgresql", database: "kanban_boards")
end
ActiveRecord::Base.establish_connection adapter: "postgresql", database: "kanban_boards"

ActiveRecord::Schema.define do
  create_table :kanban_stages do |t|
    t.string :title, null: false
    t.integer :placements_positions, null: false, array: true, default: []
  end

  create_table :kanban_stage_placements do |t|
    t.string :title, null: false
    t.references :stage, null: false, index: true
  end
end

# Played around with a general abstraction for this way, but didn't get very far with it.
class ActiveRecord::Positioned < ActiveRecord::Base
  belongs_to :record, polymorphic: true

  # name
  # position_ids, :integer, array: true

  # def ordered = record.public_send(name).in_order_of(:id, position_ids)
end

module Kanban; def self.table_name_prefix = "kanban_"; end
class Kanban::Stage < ActiveRecord::Base
  has_many :placements do
    # Mostly for debugging.
    def positioned = in_order_of(:id, proxy_association.owner.placements_positions)
  end

  def place(placement, after: nil) = transaction do
    placement.update! stage: self

    index = index(after)&.succ || 0
    update! placements_positions: placements_positions.insert(index, placement.id).uniq
  end
  def index(placement) = placements_positions.index(placement.id)
end

class Kanban::Stage::Placement < ActiveRecord::Base
  belongs_to :stage
end

todo = Kanban::Stage.create! title: "To-do"
3.times { todo.place todo.placements.create! title: "Procrastinating #{_1}" }
p todo.placements.positioned

doing = Kanban::Stage.create! title: "Doing"
doing.place doing.placements.create! title: "Bike Shedding"
doing.place todo.placements.second, after: doing.placements.last # Simulate a drag'n'drop onto the last placement
p doing.placements.positioned

binding.irb