我正確的方法在模型

Question

定義值我有一個模型：

class Cars < ActiveRecord::Base 

    tag_nr = rand(2007) 

end 

的Cars模型映射到cars表數據庫的列name，owner。

正如你在上面看到的，還有一個tag_nr，它基本上是一個的隨機數。

我想有每個實例對象Cars類的持有像上面生成一個隨機數。 但是我不想讓這個隨機數存儲在數據庫中。而在將來，我可以訪問這個實例對象通過的tag_nr：

nr = CAR_INSTANCE.tag_nr 

而且nr現在是相同的tag_nr這個汽車實例對象首先生成。

那麼，我應該在哪裏以及如何在我的汽車模型中定義這個隨機數？

Answer 1

一個簡單的方法是用after_initialize方法：（？3.0爲好）

class Cars < ActiveRecord::Base 
    after_initialize :init 

    attr_accessor :tag_nr 

    def init 
    @tag_nr = rand(2007) 
    end 
end  

這是現在在3.1回調方法：

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

Answer 2

你可以把代碼放到你的「 lib」目錄下保存爲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 

和擴展你的模型像

class Car < ActiveRecord::Base 
    extend FindRandom 

    def self.tag_nr 
    self.random 
    end 
end 

調用Car.tag_nr會爲您提供一個實例，但如果您嘗試使用其他相同類實例創建新實例，則代碼出現問題。