Ruby on Rails，数据库迁移命令和迁移任务编写

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

execute("sql")

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)