Question

我有一个模特：

class Cars < ActiveRecord::Base

  tag_nr = rand(2007)

end

Cars模型已映射到数据库中的cars表，列name，owner。

如上所述，还有tag_nr基本上是随机数。

我希望Cars 类的每个实例对象持有如上所述生成的随机数。但我不希望将此随机数存储在数据库中。将来，我可以通过：

访问此实例对象的tag_nr

nr = CAR_INSTANCE.tag_nr

nr现在与首次为此Cars实例对象生成的tag_nr 相同。

那么，我应该在哪里以及如何在我的Car模型中定义这个随机数？

Answer 1

一种简单的方法是使用after_initialize方法：

class Cars < ActiveRecord::Base
  after_initialize :init

  attr_accessor :tag_nr

  def init
    @tag_nr = rand(2007)
  end
end

现在这是3.1中的回调方法（3.0以及？）：

after_initialize do |car|
  puts "You have initialized an object!"
end

Answer 2

您可以将该代码放入＆＃39; lib＆＃39;目录另存为find_random.rb

module FindRandom
  # pull out a unique set of random active record objects without killing
  # the db by using "order by rand()"
  # Note: not true-random, but good enough for rough-and-ready use
  #
  # The first param specifies how many you want.
  # You can pass in find-options in the second param
  # examples:
  #  Product.random     => one random product
  #  Product.random(3)  => three random products in random order
  #
  # Note - this method works fine with scopes too! eg:
  #  Product.in_stock.random    => one random product that fits the "in_stock" scope
  #  Product.in_stock.random(3) => three random products that fit the "in_stock" scope
  #  Product.best_seller.in_stock.random => one random product that fits both scopes
  #
  def find_random(num = 1, opts = {})
  # skip out if we don't have any
    return nil if (max = self.count(opts)) == 0

    # don't request more than we have
    num = [max,num].min

    # build up a set of random offsets to go find
    find_ids = [] # this is here for scoping

    # get rid of the trivial cases
    if 1 == num # we only want one - pick one at random
      find_ids = [rand(max)]
    else
      # just randomise the set of possible ids
      find_ids = (0..max-1).to_a.sort_by { rand }
      # then grab out the number that we need
      find_ids = find_ids.slice(0..num-1) if num != max
    end

    # we've got a random set of ids - now go pull out the records
    find_ids.map {|the_id| first(opts.merge(:offset => the_id)) }
  end
end

并像那样扩展您的模型

class Car < ActiveRecord::Base
  extend FindRandom

  def self.tag_nr
    self.random
  end
end

调用Car.tag_nr将为您提供一个实例，但如果您尝试使用其他相同的类实例创建新实例，则代码出现问题。

我正确地定义模型中的值

2 个答案: