如果是测试数据库，直接把数据库文件删了重建就 OK 了

RAILS_ENV=development rake db:migrate

#2 楼 @amateur 报错如下

davidlee:smaple_app ft_david$ RAILS_ENV=development rake db:migrate
== 20160705132939 AddIndexToUsersEmail: migrating =============================
-- add_index(:users, :email, {:unqiue=>true})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

Unknown key: :unqiue. Valid keys are: :unique, :order, :name, :where, :length, :internal, :using, :algorithm, :type
/Users/ft_david/.rvm/gems/ruby-2.3.0/gems/activesupport-4.2.6/lib/active_support/core_ext/hash/keys.rb:75:in `block in assert_valid_keys'

很明显了，Unknown key: :unqiue. 是 unqiue 拼写错了，正确的是 unique

新手请你先学会看日志，然后学会把关键信息贴到 google 搜下。

#4 楼 @jiemoon 改了之后还是出现 ActiveRecord::PendingMigrationError 这个错误。

#5 楼 @chenjau 恩恩！