在和中对Rails中数据库迁移的概念和迁移任务的执行做了了解。其中的迁移任务仅仅停留在创建和删除表之类最简单的操作，实际开发中还有很多对数据库结构和内容的调整。下面来了解一下：
- 创建表
create_table(table,options) do |t|...columns...end
- 删除表（表名）
drop_table(table)
- 重命名表(旧表名，新表名)
rename_table(table,new_name)
- 增加列（表名，列名，列类型，属性）
add_column(table,column,type,options)
- 移除列（表名，列名）
remove_column(table,column)
- 重命名列（表名，旧列名，新列名）
rename_column(table,column,new_name)
- 修改列（表名，列名，列类型，属性）
change_column(table,column,type,options)
- 增加索引（表名，列名，属性）
add_index(table,column,options)
- 移除索引（表名，列名）
remove_index(table,column)
- 直接下发SQL命令
execute("sql") E:\greensoft\RailsInstaller\Sites\simple_cms>rails generate migration AlterUsers invoke active_record create db/migrate/20120706144844_alter_users.rb
在用于迁移的up方法中修改表名，增加一些字列，并修改列的属性。用户回滚的down方法中倒序添加反向操作。
class AlterUsers < ActiveRecord::Migration def up rename_table("users","admin_users") add_column("admin_users","username",:string,:limit=>25) add_column("admin_users","email",:string,:limit=>50) change_column("admin_users","email",:string,:default=>"",:limit=>100) add_column("admin_users","password",:string,:limit=>25) rename_column("admin_users","password","hashed_password") add_column("admin_users","salt",:string,:limit=>40) puts "***about to add an index ***" add_index("admin_users","username") end def down remove_index("admin_users","username") remove_column("admin_users","salt") rename_column("admin_users","hashed_password","password") remove_column("admin_users","password") remove_column("admin_users","email") remove_column("admin_users","username") rename_table("admin_users","users") endend 执行这个迁移任务
E:\greensoft\RailsInstaller\Sites\simple_cms>rake db:migrate== AlterUsers: migrating =====================================================-- rename_table("users", "admin_users") -> 0.0260s-- add_column("admin_users", "username", :string, {:limit=>25}) -> 0.1340s-- add_column("admin_users", "email", :string, {:limit=>50}) -> 0.1740s-- change_column("admin_users", "email", :string, {:default=>"", :limit=>100}) -> 0.1500s-- add_column("admin_users", "password", :string, {:limit=>25}) -> 0.1550s-- rename_column("admin_users", "password", "hashed_password") -> 0.1790s-- add_column("admin_users", "salt", :string, {:limit=>40}) -> 0.1540s***about to add an index ***-- add_index("admin_users", "username") -> 0.1310s== AlterUsers: migrated (1.1361s) ============================================ 改变被提交到数据库中
mysql> show fields from admin_users;+-----------------+--------------+------+-----+---------+----------------+| Field | Type | Null | Key | Default | Extra |+-----------------+--------------+------+-----+---------+----------------+| id | int(11) | NO | PRI | NULL | auto_increment || created_at | datetime | NO | | NULL | || updated_at | datetime | NO | | NULL | || username | varchar(25) | YES | | NULL | || email | varchar(100) | YES | | | || hashed_password | varchar(25) | YES | | NULL | || salt | varchar(40) | YES | | NULL | |+-----------------+--------------+------+-----+---------+----------------+7 rows in set (0.02 sec)
执行成功后可以通过回滚复原到之前的状态
E:\greensoft\RailsInstaller\Sites\simple_cms>rake db:migrate:down VERSION=20120706144844== AlterUsers: reverting =====================================================-- remove_index("admin_users", "username") -> 0.1360s-- remove_column("admin_users", "salt") -> 0.1490s-- rename_column("admin_users", "hashed_password", "password") -> 0.2400s-- remove_column("admin_users", "password") -> 0.1520s-- remove_column("admin_users", "email") -> 0.1370s-- remove_column("admin_users", "username") -> 0.1790s-- rename_table("admin_users", "users") -> 0.0260s== AlterUsers: reverted (1.0481s) ============================================ 数据库回到了最初的状态
mysql> show fields from users;+------------+----------+------+-----+---------+----------------+| Field | Type | Null | Key | Default | Extra |+------------+----------+------+-----+---------+----------------+| id | int(11) | NO | PRI | NULL | auto_increment || created_at | datetime | NO | | NULL | || updated_at | datetime | NO | | NULL | |+------------+----------+------+-----+---------+----------------+3 rows in set (0.02 sec)