Question

我遵循了概述here的内容。这是我的代码：

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

我得到了AttributeError：'Book'对象没有属性'authors'

Answer 1

ReferenceProperties将query-objects添加为引用类的属性。所以仔细看看你的映射：

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')

在您的代码中：

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

因此，b将具有books属性，而不是authors。而且，a将具有authors属性，而不是books。

如果更改集合名称，则应运行代码。

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')

此外，如果BookAuthor没有其他属性，请务必查看您引用的文章中列出的list-of-keys方法。

谷歌应用引擎中的多对多关系建模

1 个答案: