Rails: アクションによってバリデーションを切り替えた時の、モデルのテスト方法 ref: http://qiita.com/Yinaura/items/7cd94e338ae031c04820

user.rb

class User
  validates :name, presence: true, on: :index
  validates_length_of :introduction, maximum: 10000. on: :create
end

user_spec.rb

require 'spec_helper'

describe User, type: :model do
  it { should validate_presence_of(:name).on(:index) }
  it { should validate_length_of(:introduction).is_at_most(10000).on(:create) }
end