example_migration.rb

class ExampleMigration < ActiveRecord::Migration
  class MyModel < ActiveRecord::Base
    def some_method_i_need
      # ...
    end
  end
  
  def self.up
    add_column :my_models, :whatever, :string

    MyModel.find_each do |my_model|
      my_model.whatever = my_model.some_method_i_need
      my_model.save!
    end
  end

  def self.down
    remove_column :my_models, :whatever
  end
end