在活动记录关联你怎么只引用了持久记录 [英] How do you reference only the persisted records in an active record association

问题描述

在多个控制器的编辑方法初始化一个新的对象，并编辑现有的对象

In the edit method of many controllers you initialize a new object and edit existing objects

class MagazinesController < ApplicationController
   def edit
      @magazine = Magazine.find(params[:magazine_id])
      @page = Page.find(params[:id])
      @new_page = @magazine.pages.new
   end
end

然而，在一个视图中，你会经常要循环持久化对象，并单独处理的新对象。

However in a view you will often want to cycle through the persisted objects and treat the new object separately

# magazines#edit
%h4 Existing pages
- @magazine.pages.each do |page|
  %p= link_to page, page.title

问题

...是，页关联包含现有的（持续）页，而且我们通过 @new_page = @取得了新的一页magazine.pages.new 。

The problem

...is that the pages association contains both existing (persisted) pages but also the new page which we made via @new_page = @magazine.pages.new.

可以很容易地处理这种但是它的丑陋

It's easy to deal with this however it's ugly

%h4 Existing pages
- @magazine.pages.each do |page|
  - if page.persisted?
    %p= link_to page, page.title

我想用一些协会[方法来选择只有那些持续网页：

I would like to use some assocition method to select only those pages which are persisted:

%h4 Existing pages
- @magazine.pages.persisted.each do |page|
  %p= link_to page, page.title

有没有办法这样做的？

Is there any way of doing this?

推荐答案

无论从@ Florent2和@CDub的建议是合理的。然而@ florent2的建议意味着再次命中数据库（可能抛弃任何preset的，我不想做预先加载）和@ CDub的建议并没有完全制定出在code条款。以下是我结束了去：

Both the suggestions from @Florent2 and @CDub are sound. However @florent2's suggestion meant hitting the database again (and potentially ditching any preset eager loading which I didn't want to do) and @CDub's suggestion didn't quite work out in terms of code. Here is what I ended up going with:

class Magazine < ActiveRecord::Base
  has_many :pages do 
    def persisted
      collect{ |page| page if page.persisted? }
    end
  end
end

这允许你调用 .persisted 在与杂志​​有关的网页的任何ActiveRecord的关系。它不会再次访问数据库，因为它只需通过pre-加载的对象返回它们坚持的那些过滤器。

this allows you to call .persisted on any ActiveRecord relation of pages associated with Magazine. It doesn't hit the database again as it simply filters through the pre-loaded objects returning the ones which are persisted.

因为我想重新使用此code定期，我可以拉出来变成一个模块

Since I want to reuse this code on a regular basis I can pull it out into a module

module PersistedExtension
  def persisted
    select{|item| item if item.persisted?}
  end
end

有然后可以纳入使用一个lambda关联方法：

It can then be included into the association methods using a lambda:

class Magazine < ActiveRecord::Base
  # ...
  has_many :pages, -> { extending PersistedExtension }

end

和我能直观地叫它：

@magazine = Magazine.first

@magazine.pages.persisted
# => array of pages which are persisted

# the new persisted association extension works on any AR result set
@magazine.pages.order('page ASC').persisted

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