使用選項

:on 選項允許你指定驗證何時發生。所有內建驗證助手的預設行為都是在儲存時執行的（無論是在建立新記錄時還是在更新時）。

class Person < ApplicationRecord
  # it will be possible to update email with a duplicated value
  validates :email, uniqueness: true, on: :create
 
  # it will be possible to create the record with a non-numerical age
  validates :age, numericality: true, on: :update
 
  # the default (validates on both create and update)
  validates :name, presence: true
end