Example of migration

alter_user_migration.rb

class AlterUser < ActiveRecord::Migration
        def up
                rename_table :users, :customers
                add_column :customers, :username, :string, limit: 25
                change_column :customers, :email, :string, limit: 100
                rename_column :customers, :password, :hashed_password
                add_column :customers, :salt, :string, limit: 40
                add_index :customers, :username
        end
 
        def down
                rename_table :customers, :users
                remove_column :users, :username
                change_column :users, :email, :string, default: "", null: false  # you should specify field type after column name as the third argument in the change_column funtion
                rename_column :users, :hashed_password, :password
                remove_column :users, :salt
        end
end