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http://guides.rubyonrails.org/v3.0.8/active_record_validations_callbacks.html#on

在特定的action里使用validates

The :on option lets you specify when the validation should happen. The default behavior for all the built-in validation helpers is to be run on save (both when you’re creating a new record and when you’re updating it). If you want to change it, you can use :on => :create to run the validation only when a new record is created or :on => :update to run the validation only when a record is updated.

class Person < ActiveRecord::Base
  # it will be possible to update email with a duplicated value
  validates_uniqueness_of :email, :on => :create
 
  # it will be possible to create the record with a non-numerical age
  validates_numericality_of :age, :on => :update
 
  # the default (validates on both create and update)
  validates_presence_of :name, :on => :save
end