问Rails 4 before_create方法未保存到DB

EN

class Distribution < ActiveRecord::Base
  belongs_to :user
  #validates :distribution_digest, presence: true
  attr_accessor :distribution_token, :distribution_digest
  before_create :create_distribution_digest
  validates :signature_id, presence: true

  def Distribution.digest(string)
    cost = ActiveModel::SecurePassword.min_cost ? BCrypt::Engine::MIN_COST :
                                                  BCrypt::Engine.cost
    BCrypt::Password.create(string, cost: cost)
  end

  # Returns a random token.
  def Distribution.new_token
    SecureRandom.urlsafe_base64
  end

  private

    # Creates and assigns the distribution token and digest.
    # Functions create tokens and hashed tokens as expected.
    def create_distribution_digest
      self.distribution_token  = Distribution.new_token
      self.distribution_digest = Distribution.digest(distribution_token)
    end
end

class DistributionsController < ApplicationController
  before_action :logged_in_user, only: [:email_sig]

  def create
    if current_user.microposts.find(params[:signature_id])
      @distribution = current_user.distributions.build(distribution_params)
      @distribution.save
      respond_with current_user.microposts.find(params[:signature_id])
    end
  end

  private

    def distribution_params
      params.require(:distribution).permit(:signature_id).merge(:distributed_at => Time.zone.now)
    end

end

Started POST "/emailSig.json" for 180.181.247.76 at 2015-12-09 05:01:47 +0000
Processing by DistributionsController#create as JSON
  Parameters: {"signature_id"=>8, "distribution"=>{"signature_id"=>8}}
  User Load (0.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1  [["id", 2]]
  Micropost Load (3.2ms)  SELECT  "microposts".* FROM "microposts" WHERE "microposts"."user_id" = ? AND "microposts"."id" = ?  ORDER BY "microposts"."created_at" DESC LIMIT 1  [["user_id", 2], ["id", 8]]
   (0.2ms)  begin transaction
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  SQL (1.0ms)  INSERT INTO "distributions" ("signature_id", "distributed_at", "user_id", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["signature_id", "8"], ["distributed_at", "2015-12-09 05:01:48.469044"], ["user_id", 2], ["created_at", "2015-12-09 05:01:48.484218"], ["updated_at", "2015-12-09 05:01:48.484218"]]
   (11.8ms)  commit transaction

  Distribution.last
  Distribution Load (0.4ms)  SELECT  "distributions".* FROM "distributions"  ORDER BY "distributions"."id" DESC LIMIT 1
=> #<Distribution id: 27, user_id: 2, signature_id: "8", distribution_digest: nil, distributed_at: "2015-12-09 05:01:48", created_at: "2015-12-09 05:01:48", updated_at: "2015-12-09 05:01:48">