2.rb

require 'pry'

class AddExternalSystemId < ActiveRecord::Migration

  def up
  	Rails.application.paths["app/models"].each do |model_path|
  	  Dir["#{Rails.root}/#{model_path}/*/*.rb"].each { |file| require file }
  	end
  	models = ActiveRecord::Base.send(:subclasses)
  	binding.pry
  	tables = models.delete_if {|a| a.to_s =~ /Cmi::MdmRelations|Irm::GlobalSetting|Delayed::Backend::ActiveRecord::Job|ActiveRecord::SessionStore::Session/i }.delete_if {|a| a.column_names.include?("external_system_id") }.map {|a| a.table_name }
  	binding.pry
  	tables.each do |table_name|
  		begin
		  	add_column table_name, :external_system_id, :string, :limit => 32
		  rescue Mysql2::Error
		    binding.pry
		    puts table_name
		    next
		  end
  	end
  end

  def down
  	tables = ActiveRecord::Base.connection.tables.map {|a| a.to_sym }
  	binding.pry
  	tables.each do |table_name|
  		begin
	  		remove_column table_name, :external_system_id
	  	rescue ActiveRecord::StatementInvalid
	  		next
	  	end
  	end
  end

end