Database Rake aborted... table ‘users’ already exists

我已经用DEVISE和漂亮的生成器创建了一个数据库.我正在try 使用漂亮的生成器(rails g nifty:scaffold Asset user_id:integer)创建一个新数据库，但是当我try 迁移数据库(rake db:migrate)时，我得到以下错误:

charlotte-dator:showwwdown holgersindbaek$ rake db:migrate
==  DeviseCreateUsers: migrating ==============================================
-- create_table(:users)
rake aborted!
An error has occurred, all later migrations canceled:

Mysql2::Error: Table 'users' already exists: CREATE TABLE `users` (`id` int(11) DEFAULT NULL auto_increment PRIMARY KEY, `email` varchar(255) DEFAULT '' NOT NULL, `encrypted_password` varchar(128) DEFAULT '' NOT NULL, `reset_password_token` varchar(255), `reset_password_sent_at` datetime, `remember_created_at` datetime, `sign_in_count` int(11) DEFAULT 0, `current_sign_in_at` datetime, `last_sign_in_at` datetime, `current_sign_in_ip` varchar(255), `last_sign_in_ip` varchar(255), `name` varchar(255), `created_at` datetime, `updated_at` datetime) ENGINE=InnoDB

Tasks: TOP => db:migrate
(See full trace by running task with --trace)

我正在学习一个教程，很难理解为什么会发生这种情况.有人能解释一下这是怎么回事吗？