Rails Migration from 1-M to M-M Relationship

01_create_instances.rb

class CreateInstances < ActiveRecord::Migration
  def self.up
    # Create the new instances table
    create_table :instances do |t|
      t.references :zone
      t.references :raid
      t.datetime :start_time
      t.datetime :end_time

      t.timestamps
    end

    # Create the drops.instance relationship
    add_column :drops, :instance_id, :integer
    Drop.reset_column_information

    # Create the instances.players relationship
    create_table :instances_players, :id => false do |t|
      t.references :instance
      t.references :player
    end
  end

  def self.down
    remove_column :drops, :instance_id
    Drop.reset_column_information

    drop_table :instances_players
    drop_table :instances
  end
end

02_populate_instances.rb

class PopulateInstances < ActiveRecord::Migration
  def self.up
    Raid.all.each do |raid|
      instance = Instance.create(:zone => raid.zone,
                                 :start_time => raid.start_time,
                                 :end_time => raid.end_time)

      Drop.find_all_by_raid_id(raid.id).each do |drop|
        drop.instance_id = instance.id
        drop.save
      end
      raid.players.each { |player| instance.players << player }
      instance.save
    end
  end

  def self.down
    Instance.delete_all
    Drop.each.all do |drop|
      drop.instance_id = nil
      drop.save
    end
  end
end

03_restructure_raids.rb

class RestructureRaids < ActiveRecord::Migration
  def self.up
    Raid.delete_all

    Instance.all.each do |instance|
      raid = Raid.find_by_raid_date(instance.start_time.to_date)
      raid ||= Raid.create!(:raid_date => instance.start_time.to_date)
      instance.raid_id = raid.id
      instance.save
    end

    remove_column :raids, :zone_id
    remove_column :raids, :start_time
    remove_column :raids, :end_time
    Raid.reset_column_information

    remove_column :drops, :raid_id
    Drop.reset_column_information

    drop_table :players_raids
  end

  def self.down
    add_column :drops, :raid_id, :integer
    Drop.reset_column_information

    create_table :players_raids, :id => false do |t|
      t.integer :player_id
      t.integer :raid_id
    end

    add_column :raids, :zone_id, :integer
    add_column :raids, :start_time, :datetime
    add_column :raids, :end_time, :datetime
    Raid.reset_column_information

    Raid.delete_all

    Instance.all.each do |instance|
      raid = Raid.create!(:zone => instance.zone,
                          :raid_date => instance.start_time.to_date,
                          :start_time => instance.start_time,
                          :end_time => instance.end_time)
      raid.players << instance.players
      raid.drops << instance.drops
      raid.save
    end
  end
end

raid.rb

class Raid < ActiveRecord::Base
  belongs_to :zone
end

raid_v2.rb

class Raid < ActiveRecord::Base
  has_many :zones, :through => :raids_zones
end

zone.rb

class Zone < ActiveRecord::Base
  has_many :raids
end

zone_v2.rb

class Zone < ActiveRecord::Base
  has_many :raids, :through => :raids_zones
end