rails before_create不触发方法

我试图在保存实例之前立即触发方法。我有User模型：

class User < ActiveRecord::Base

    has_secure_password
    attr_accessible :name, :first_surname,:second_surname,:email, :password, :password_confirmation,:number,:credit

    before_save{ self.email.downcase! }
    before_create :generate_auth_token

    default_scope order: 'users.created_at ASC'

    has_many :operations

    def consume(what,price,agent)
        self.operations.create(category:what,price:price, agent_id:agent)
    end
end

而每User有许多Operation（注意，这里使用的撬通过debuger binding.pry：

class Operation < ActiveRecord::Base
  attr_accessible :agent_id, :comment, :postcredit, :precredit, :category, :user_id,:price
  validates_presence_of :user_id
  validates_presence_of :agent_id
  validates_presence_of :price
  validates_presence_of :precredit
  validates_presence_of :postcredit
  validates_presence_of :category
  #before_save :compute_prices, :on => :create
  before_create :compute_prices
  belongs_to :user

  private

  def compute_prices
      binding.pry
      user=User.find(self.user_id)
      self.precredit=user.credit
      #select whether adding or subtracting
      if self.category == 'credit'
          self.postcredit=self.precredit+self.price
      else
          self.postcredit=self.precredit-self.price
      end
      user.update_attributes(credit:self.postcredit)
  end
end

我用用户和操作填充数据库，并通过控制台进行测试$rails c --sandbox。然后我：

>fi=User.first
>ope=fi.operations.create(category:'credit',price:12.2,agent_id:3)
#Now here the debugger should start and does not

我同时尝试了before_create和before_save，但均无济于事。

before_create :compute_prices
before_save :compute_prices, :on => :create

唯一可行的选择是after_initialize :compute_prices，但这在每次find初始化后都会触发。

有任何想法吗？

解

如对第一个答案的评论所述，解决方案是针对用户before_validation (function), on: :create，而不是before_save ...。