谷歌應用引擎中的多對多關係建模

Question

我遵循了概述的here。這裏是我的代碼：

from google.appengine.api import users 
from google.appengine.ext import db 


class Book(db.Model): 
    title = db.StringProperty() 

class Author(db.Model): 
    name = db.StringProperty() 

class BookAuthor(db.Model): 
    book = db.ReferenceProperty(Book, required=True, collection_name='books') 
    author = db.ReferenceProperty(Author, required=True, collection_name='authors') 

b = Book(title="My Book") 
a = Author(name="Author of My Book") 

db.put([b, a]) 

ba = BookAuthor(book=b, author=a) 
ba.put() 

b.authors 
a.books 

，我也得到AttributeError的：「書」對象有沒有屬性「作者」

Answer 1

ReferenceProperties添加query-objects的屬性中引用的類。所以，仔細看看你的映射：

class BookAuthor(db.Model): 
    # This adds a query-object as an attribute named 'books' to Book entities. 
    book = db.ReferenceProperty(Book, required=True, collection_name='books') 
    # This adds a query-object as an attribute named 'authors' to Author entities. 
    author = db.ReferenceProperty(Author, required=True, collection_name='authors') 

在您的代碼：

b = Book(title="My Book") 
a = Author(name="Author of My Book") 

所以，b將有一個books屬性，而不是authors。而且，a將具有authors屬性，而不是books。

如果更改集合名稱，應該運行您的代碼。

class BookAuthor(db.Model): 
    # This adds a query-object as an attribute named 'authors' to Book entities. 
    book = db.ReferenceProperty(Book, required=True, collection_name='authors') 
    # This adds a query-object as an attribute named 'books' to Author entities. 
    author = db.ReferenceProperty(Author, required=True, collection_name='books') 

而且，如果BookAuthor沒有附加屬性，一定要看看你提到的文章中概述的列表的密鑰的方法。