在《Ruby on Rails，rake工具使用和数据库migrations迁移的概念》和《Ruby on Rails，创建和执行migrations迁移文件》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
几个例子试试手，之前通过创建user模型操作创建了一个空的user表。现在创建一个修改users表的迁移任务。为迁移文件命名最好表明动作的含义，以后回来查找方便一些。
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")
end
end
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)
