migration.rb

# SQLiteを使っていてカラム削除の migration を作るとこんな感じ？
class RemoveCFromT1 < ActiveRecord::Migration
  def up
    execute "
       CREATE TEMPORARY TABLE t1_backup(a,b);
       INSERT INTO t1_backup SELECT a,b FROM t1;
       DROP TABLE t1;
       CREATE TABLE t1(a,b);
       INSERT INTO t1 SELECT a,b FROM t1_backup;
       DROP TABLE t1_backup;"
  end
  
  def down
    add_column :t1, :c, :integer
  end
end