向表中添加唯一列

要向 users 添加新的唯一列 email，请运行以下命令：

 placeholderCopyrails generate migration AddEmailToUsers email:string:uniq

这将创建以下迁移：

 placeholderCopyclass AddEmailToUsers < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :email, :string
    add_index :users, :email, unique: true
  end
end