GAE：模型失去父级>子关系的跟踪 [英] GAE: Model loses track of parent->child relationship

问题描述

我在google app引擎数据存储中遇到了一个非常奇怪的Entity关系问题。我在一个Python / GAE webapp（学习练习），完整的代码可以在 sourceforge 。

I'm having what seems like a very strange problem with an Entity relationship in the google app engine data store. I'm work on a Python/GAE webapp (learning exercise), the full code to which can be found on sourceforge.

• 我有2个型号：
  
  
  
  • 图库 - 搜索字词和（间接）照片列表
  
  
  • 照片 - 有关照片的信息，
  
  
  
  • 我有一个获取过程，可以创建图库并向其中添加照片
  
  
  • 我有一个从数据存储区读取图库的页面，并使用该图库实例的.photos属性来获取其中的照片列表。
  
  
  • I have 2 models:
    • Gallery - a search term and (indirectly) a list of photos
    • Photo - information about a photo, plus the gallery it belongs to (collection_index='photos')

    现在这是奇怪的部分来的地方... 如果我更改一个文件（我测试过的任何文件），或者只是更新文件的时间戳例如，如果我尝试加载，那么它会被重新加载。 flowersrel =nofollow>花画廊的页面：

    Now this is where the weird part comes in... If I change a file (any file I've tested on) or even just update the timestamp of the file (ie, so it gets reloaded)... the ".photos" attribute of galleries starts failing. For example, if I try to load the page for the "flowers" gallery:

    Traceback (most recent call last):
      File "C:\Applications\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 700, in __call__ handler.get(*groups)
      File "C:\Eclipse-Workspace\galleries-google\app\views\gallery.py", line 33, in get for photo in gallery.photos:
    AttributeError: 'Gallery' object has no attribute 'photos'
    

    我可以重新启动webapp，然后启动webapp。无论如何，问题仍然存在。看来我需要强制数据存储以某种方式记住连接

    I can restart the webapp, I can restart the app launcher and then start the webapp. The problem persists regardless of this. It seems I need to force the datastore to "remember" the connection in some way

    # Reading the list of photos for a given gallery via the Photo entity
    # This seems to force the datastore to "remember" the connection between the two
    from google.appengine.ext import db
    import pprint
    from app.models.gallery import Gallery
    from app.models.photo import Photo
    
    gallery = Gallery.get_by_key_name('candy')
    print("Gallery: " + gallery.search_term)
    q = db.GqlQuery("SELECT * FROM Photo WHERE gallery = :1", gallery )
    photos = q.fetch(20)
    
    for photo in photos :
        print("\tphoto: " + photo.getUrl('original'))
    

    通过从头重新获取所有的数据（虽然我想，即使只是重新摄取一个画廊会做）。

    or by re-ingesting all the data from scratch (though I suppose even just re-ingesting a single gallery would do).

    有没有任何想法

    Does anyone have any thoughts as to what might be causing this? Any help would be appreciated.

    注意：

    
    
    • 这是在开发环境中。
    
    
    • 奇怪的是，我最近问了一个关于如何如何在GAE中列出父节点中的子节点...因为我真的想要该信息存在于父模型定义中。
    • This is in the dev environment. I haven't gotten to the point where it's worth registering it as a webapp on the real servers yet.
    • Oddly enough, I recently asked a question on how to How to list child nodes in parent in GAE... since I really wanted the information to exist in the parent model definition. Ironic that it's (apparently) a lack of that information in the parent that's causing a problem now

    推荐答案

    p>因此，经过多次挖掘，我相信我找到了一个答案...

    • When a file is modified (the timestamp changes), the app engine invalidates the compiled/cached files it has
    • When the Photo class is defined, it adds a property to the Gallery based on the collection of ids it has for them
    • When it needs the Gallery class, the app engine reloads the code for it... because it's definition was invalidated by a file being modified
    • Since it doesn't know it needs the Photo class, it doesn't load that file, so the "new instance" of the Gallery class doesn't have the property that should be created for it's collection
    
    
    • 当文件被修改，应用程序引擎会使已编译/缓存的文件失效
    
    
    • 定义 Photo 类时， c $ $ >类，应用程序引擎重新加载代码...因为它的定义被正在修改的文件无效
    
    
    • 因为它不知道它需要照片类，它不会加载该文件，因此 Gallery 类的新实例没有应该

    My first solution was to add a dependency on the Photo file in the Gallery file.

    from app.models.photo import Photo
    

    这样的问题是在两个文件之间创建一个循环依赖（因为照片已经需要知道图库以获得它们的列表）。虽然它工作，我被告知Python有问题与循环依赖，这可能不是一件好事;

    The problem with this is that it creates a circular dependency between the two files (since Photo already needs to know about Gallery in order to have a list of them). While it worked, I was told that Python has "problems" with circular dependencies, and it's probably not a good thing to do; it may come back to bite me later.

    我解决的解决方案是将__init.py 文件所讨论的包包括两个文件的依赖关系。因此，任何时候只需要其中一个文件即可加载。

    The solution I wound up going with was to have the __init.py file for the package in question include a dependency for both files. As such, any time either one of the files is needed, both will be loaded.

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