Django模型继承和管理系统 [英] Django Model Inheritance and Admin System

问题描述

我正在尝试构建一个管理页面中各种内容的系统。例如，页面可以具有文本内容，超链接内容，视频内容等等。

在我的建模代码中，我有一个基类：

  ContentItem（models.Model）：
 title = models.CharField（max_length = 1000）
 page_order = models.IntegerField（）
 last_update_date = models.DateTimeField（default = datetime.now（） ）
 
 class Meta：
 abstract = True 
 order = ['page_order'，'last_update_date'，'title'] 
  / pre> 
 
 
这是所有内容项的基类。页面顺序控制页面上的位置，例如page_order = 0的项目应位于页面顶部。接下来，我定义了一些继承自此的特定内容模型。
  class LinkContent（ContentItem）：
 url = models.URLField（）
 link_text = models.CharField（max_lenth = 1000）
 
 class TextContent（ContentItem）：
 text = models.CharField（）
 
 
 class VideoContent（ContentItem）：
 title = models.CharField（）
 video_file = models.FieldField（upload_to ='videos'）
  
可能还有更多这样的内容类型。然后我将定义一个由所有各种内容类型组成的Page模型。理想情况下，我可以根据基本类型将所有类型放在关系上。所以在这个关系中，你将会混合使用LinkContents，TextContents和VideoContents。他们将按page_order排序，以便在呈现模板时确定页面上的顺序。
 类页面（models.Model）： 
 contents = models.ManyToManyField（ContentItem）
 title = models.CharField（）
  
 
 
 <有没有办法使这样的计划工作？或者与其中不同类型的模型有一个关系是有问题的吗？我知道这是一个很好的解决方案和面向对象编程的立场，基本上是使用多态来实现我的优势，但我不确定它在数据库级别是有意义的。
 
 
 
我更需要一些这样的东西：
  class Page（models.Model）：
 video_contents = models.ManyToManyField VideoContent）
 link_contents = models.ManyToManyField（LinkContent）
 text_contents = models.ManyToManyField（TextContent）
 title = models.CharField（）
  pre> 
 
 
我知道这可以工作，但是我确定页面上对象放置的方案变得更加困难。我需要遍历所有的内容关系，用page_order排序，然后渲染它们。 
 
 
 
我认为在这两种情况下，我想在基类上声明每个特定内容类型可以继承的render（）方法。这样，如果我有一个ContentItem列表，我可以使用鸭子打字来渲染它们，而不用担心他们的具体类型。
 
 
 
我的最后一个问题是如何让管理员放置好有了这个？如何让一个简单的方法来查看在一个视图中构成页面的所有ContentItem，这样可以通过更改page_order来轻松地移动它。
 
 
 
感谢阅读这个，让我知道，如果你需要更多的信息。
解决方案
这是一个很好的方法。不幸的是，Django的ORM不像你想要的那样顺利地处理模型继承。  page.contents 将包含一个内容对象的QuerySet。如果要访问子类型，则需要创建一些缩减内容对象的方法。问题是这需要每个对象的查询，这可以快速失控。 此博文描述了一种技术使用 select_related（）在一个queryset中获取混合子类型。 
 
I'm trying to build a system for managing the various types of content in a page.  For example, a page may have text content, hyperlink content, video content, ect.  

In my modeling code, I have a base class:
class ContentItem(models.Model):
    title = models.CharField(max_length=1000)
    page_order = models.IntegerField()
    last_update_date = models.DateTimeField(default=datetime.now())

    class Meta:
      abstract = True
      ordering = ['page_order', 'last_update_date', 'title']
This is the base class for all content items.  The page order controls what position it is on the page, for example the item with page_order = 0 should be at the top of the page.  Next I define a few specific content models that inherit from this one.
class LinkContent(ContentItem):
  url = models.URLField()
  link_text = models.CharField(max_lenth=1000)

class TextContent(ContentItem):
 text = models.CharField()


class VideoContent(ContentItem):
      title = models.CharField()
      video_file = models.FieldField(upload_to = 'videos')
There could be many more such content types.  Then I would define a Page model that is composed of all the various content types.  Ideally, I could put all the types in on relation based on the base type.  So in this one relation you would have a mixture of LinkContents, TextContents, and VideoContents.  They would be sorted by page_order to determine their order on the page when rendering the template.
class Page(models.Model):
  contents = models.ManyToManyField(ContentItem)
  title = models.CharField()
Is there any way to make such a scheme work?  Or is it problematic to have one relation with different types of models in it?  I know this is a good solution from and object oriented programming standpoint, basically using polymorphism to my advantage, but I am not sure it makes sense at the database level.

Do I instead need something more like this:
class Page(models.Model):
  video_contents = models.ManyToManyField(VideoContent)
  link_contents = models.ManyToManyField(LinkContent)
  text_contents = models.ManyToManyField(TextContent)
  title = models.CharField()
I know this would work, but my scheme of determining the placement of the objects on the page becomes more difficult.  I would need to traverse all the content relations, sort them by page_order and then render them.  

I think in both cases, I want to declare a render() method on the base class that each specific content type can inherit.  This way if I have a list of ContentItems I can use duck typing to render them without worrying about their specific type.

My final question is how do I make admin place nice with this?  How would I make an easy way to see all of the ContentItems that make up a page in one view, so they can easily be moved around by changing page_order?

Thanks for reading of this, let me know if you need more information.
解决方案
This is a fine way to do it.  Unfortunately, Django's ORM doesn't handle model inheritance as smoothly as you might want.  page.contents will contain a QuerySet of Content objects. If you want to access the subtypes, you need to create some way of downcasting a content object.  The problem is that this requires an query per object, which can rapidly get out of hand.  This blog post describes a technique for getting the mixed subtypes in one queryset, using select_related() behind the scenes.  

                        
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