由于 GAE 的数据存储是非关系的，因此您必须在不使用 joins 的情况下为您的 多对多 关系建模。这里有两种方法可以轻松适应您的需求。
使用链接模型方法的工作示例（更新 #1）
from google.appengine.ext import db
# Defining models
class User(db.Model):
name = db.StringProperty()
class Software(db.Model):
name = db.StringProperty()
description = db.TextProperty()
class UserSoftwares(db.Model):
user = db.ReferenceProperty(User, collection_name='users')
software = db.ReferenceProperty(Software, collection_name='softwares')
# Creating users
u1 = User(name='John Doe')
u2 = User(name='Jane Doe')
# Creating softwares
sw1 = Software(name='Office 2007')
sw2 = Software(name='Google Chrome')
sw3 = Software(name='Notepad ++')
# Batch saving entities
db.put([u1, u2, sw1, sw2, sw3])
"""
Creating relationship between users and softwares;
in this example John Doe's softwares are 'Office 2007' and
'Notepad++' while Jane Doe only uses 'Google Chrome'.
"""
u1_sw1 = UserSoftwares(user=u1, software=sw1)
u1_sw3 = UserSoftwares(user=u1, software=sw3)
u2_sw2 = UserSoftwares(user=u2, software=sw2)
# Batch saving relationships
db.put([u1_sw1, u1_sw3, u2_sw2])
"""
Selects all softwares.
"""
rs1 = Software.all()
# Print results
print ("SELECT * FROM Software")
for sw in rs1:
print sw.name
"""
Selects a software given it's name.
"""
rs2 = Software.all().filter("name =", "Notepad ++")
# Print result
print("""SELECT * FROM Software WHERE name = ?""")
print rs2.get().name
"""
Selects all software used by 'John Smith'.
"""
# Get John Doe's key only, no need to fetch the entire entity
user_key = db.Query(User, keys_only=True).filter("name =", "John Doe").get()
# Get John Doe's software list
rs3 = UserSoftwares.all().filter('user', user_key)
# Print results
print ("John Doe's software:")
for item in rs3:
print item.software.name
"""
Selects all users using the software 'Office 2007'
"""
# Get Google Chrome's key
sw_key = db.Query(Software, keys_only=True).filter("name =", "Google Chrome").get()
# Get Google Chrome's user list
rs4 = UserSoftwares.all().filter('software', sw_key)
# Print results
print ("Google Chrome is currently used by:")
for item in rs4:
print item.user.name
链接模型方法（推荐）
您可以通过以这种方式表示每个表来为 多对多 关系建模：
from google.appengine.ext import db
class User(db.Model):
name = db.StringProperty()
class Software(db.Model):
name = db.StringProperty()
description = db.TextProperty()
class UserSoftwares(db.Model):
user = db.ReferenceProperty(User, collection_name='users')
software = db.ReferenceProperty(Software, collection_name='softwares')
如您所见，它与关系型的思维方式非常相似。
键列表方法（备选）
关系也可以建模为键列表：
class User(db.Model):
name = db.StringProperty()
softwares = db.ListProperty(db.Key)
class Software(db.Model):
name = db.StringProperty()
description = db.TextProperty()
@property
def users(self):
return User.all().filter('softwares', self.key())
这种方法更适合少量键，因为它使用 ListProperty，但比上面的 链接模型方法更快。