Add/Rename a column to/in a table in Rails ActiveRecords

Let's consider that you already have a Users table

$ rails generate migration AddEmailToUsers

This will create the following file

db/migrate/some_timestamp_add_email_to_users.rb

Edit this file as the following

class AddEmailToUsers < ActiveRecord::Migration

  def change
    #to add a new column

    add_column :users, :email, :string
    # to rename an existing column
          rename_column :users, :username, :user_name

  end

end

And finally run

$ rake db:migrate

Check your database, a new column "email" was added to all users tables.