如何从另一个引用相同的模型两次？

Question

下面的代码

class Translation(db.Model): 
    origin = db.ReferenceProperty(Expression, required=True) 
    target = db.ReferenceProperty(Expression, required=True) 

产生以下错误：

Traceback (most recent call last): File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 4053, in _HandleRequest self._Dispatch(dispatcher, self.rfile, outfile, env_dict) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 3977, in _Dispatch base_env_dict=env_dict) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 588, in Dispatch base_env_dict=base_env_dict) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 3050, in Dispatch self._module_dict) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2954, in ExecuteCGI reset_modules = exec_script(handler_path, cgi_path, hook) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2834, in ExecuteOrImportScript exec module_code in script_module.dict File "D:\svn\language\Web\src\controller.py", line 5, in from model import * File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1505, in Decorate return func(self, *args, **kwargs) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2450, in load_module return self.FindAndLoadModule(submodule, fullname, search_path) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1505, in Decorate return func(self, *args, **kwargs) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2339, in FindAndLoadModule description) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 1505, in Decorate return func(self, *args, **kwargs) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\tools\dev_appserver.py", line 2282, in LoadModuleRestricted description) File "D:\svn\language\Web\src\model.py", line 24, in class Translation(db.Model): File "C:\Program Files (x86)\Google\google_appengine\google\appengine\ext\db__init__.py", line 500, in init _initialize_properties(cls, name, bases, dct) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\ext\db__init__.py", line 415, in _initialize_properties attr.property_config(model_class, attr_name) File "C:\Program Files (x86)\Google\google_appengine\google\appengine\ext\db__init__.py", line 3461, in property_config self.collection_name)) DuplicatePropertyError: Class Expression already has property translation_set

如何规避这种限制？

Answer 1

class Translation(db.Model): 
    origin = db.ReferenceProperty(Expression, required=True, collection_name='translation_origins') 
    target = db.ReferenceProperty(Expression, required=True, collection_name='translation_targets') 

Answer 2

给他们不同的集合名称：

class Translation(db.Model): 
    origin = db.ReferenceProperty(Expression, required=True, 
            collection_name='origin_translation_set') 
    target = db.ReferenceProperty(Expression, required=True, 
            collection_name='target_translation_set') 

Answer 3

给不同的集合名称

class Translation(db.Model): 
    origin = db.ReferenceProperty(Expression, required=True,collection_name='origin_translation_set') 
    target = db.ReferenceProperty(Expression, required=True,collection_name='target_translation_set') 

每db.ReferenceProperty默认创建的模型被称为一个名为referencedmodelname_set集合。

对于如：

class OwnedCar(db.Model): 
    brand = db.StringProperty(required=True) 
    owner = db.ReferenceProperty(Human, required=True) 

class Human(db.Model): 
    name = db.StringProperty(required=True) 
    drives = db.ReferenceProperty(reference_class=Car) 

默认情况下，每个人都会有默认名称ownedcar_set的属性。如果您指的是同一个模型两次，则会创建collection_name冲突。所以你应该添加collection_name来区分集合。

更多参考here