我正确的方法在模型

Question

定义值我有一个模型：

class Cars < ActiveRecord::Base 

    tag_nr = rand(2007) 

end 

的Cars模型映射到cars表数据库的列name，owner。

正如你在上面看到的，还有一个tag_nr，它基本上是一个的随机数。

我想有每个实例对象Cars类的持有像上面生成一个随机数。 但是我不想让这个随机数存储在数据库中。而在将来，我可以访问这个实例对象通过的tag_nr：

nr = CAR_INSTANCE.tag_nr 

而且nr现在是相同的tag_nr这个汽车实例对象首先生成。

那么，我应该在哪里以及如何在我的汽车模型中定义这个随机数？

Answer 1

一个简单的方法是用after_initialize方法：（？3.0为好）

class Cars < ActiveRecord::Base 
    after_initialize :init 

    attr_accessor :tag_nr 

    def init 
    @tag_nr = rand(2007) 
    end 
end  

这是现在在3.1回调方法：

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

Answer 2

你可以把代码放到你的“ lib”目录下保存为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 

和扩展你的模型像

class Car < ActiveRecord::Base 
    extend FindRandom 

    def self.tag_nr 
    self.random 
    end 
end 

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