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POInheriting AbstractConcreteBase multiple times [was: Adding an Auto-ID field to a model base class via mixin]
 

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  1. POInheriting AbstractConcreteBase multiple times [was: Adding an Auto-ID field to a model base class via mixin]
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    <p>I have created an <code>ModelBase</code> class for an application of mine based on SQLAlchemy.</p> <p>To save me the tedious work of having to type out <code>id = Column(Integer, primary_key=True)</code> I provided the attribute as default id on my <code>ModelBase</code> class.</p> <pre class="lang-python prettyprint-override"><code>Base = declarative_base() class ModelBase(AbstractConcreteBase, Base): id = Column(Integer, primary_key=True) def __init__(self, *args, **kwargs): """ constructor """ # do some nifty magic, keyword cleaning on kwargs, as we might have data # coming from json input super(ModelBase,self).__init__(*args, **kwargs) def do_sth_else(self): """ some shared fundamental logic, json (de)serialization """ </code></pre> <p>I probably should't have done this, because now all classes do have an id integer field. Turns out, I want to use composite keys on some models, but I still want to have the default of a model class having <code>id</code> as primary key. So I decided to write a mixin class and provide different ModelBase classes.</p> <pre class="lang-python prettyprint-override"><code>Base = declarative_base() class IDMixin(object): id = Column(Integer, primary_key=True) class AbstractModelBase(object): def __init__(self, json_data='', *args, **kwargs): """ same constructor """ super(AbstractModelBase,self).__init__(*args, **kwargs) def do_sth_else(self): """ same shared fundamental logic """ class ModelBase(AbstractConcreteBase, Base, IDMixin, AbstractModelBase): """ new model base class """ class NoIDModelBase(AbstractConcreteBase, Base, AbstractModelBase): """ new model base class """ </code></pre> <p>However, instantiating this class with a keyword dictionary gave me a painful stack trace:</p> <pre><code>$ ./manage.py test # no, not django ;) # Test 1 of 30 =============== Traceback (most recent call last): File "./manage.py", line 40, in &lt;module&gt; execute_command(sys.argv) File "./manage.py", line 36, in execute_command cmd(argv[2:]) File "./management/test.py", line 362, in handle DBtestRunner(verbosity=args.verbosity).run(tests) File "./management/test.py", line 172, in run setUpDB(t) File "./management/test.py", line 134, in setUpDB instance = model_class(**d) ### instantiation occurs here ### File "&lt;string&gt;", line 2, in __init__ File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 310, in _new_state_if_none state = self._state_constructor(instance, self) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 582, in __get__ obj.__dict__[self.__name__] = result = self.fget(obj) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 145, in _state_constructor self.dispatch.first_init(self, self.class_) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/event.py", line 409, in __call__ fn(*args, **kw) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2197, in _event_on_first_init configure_mappers() File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2123, in configure_mappers _call_configured.dispatch.after_configured() File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/event.py", line 372, in __call__ fn(*args, **kw) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/orm/events.py", line 489, in wrap wrapped_fn(*arg, **kw) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 51, in go cls.__declare_last__() File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 347, in __declare_last__ cls.__mapper__ = m = mapper(cls, pjoin, polymorphic_on=pjoin.c.type) File "/path/to/code/refactoring/.env/lib/python2.7/site-packages/sqlalchemy/util/_collections.py", line 172, in __getattr__ raise AttributeError(key) AttributeError: type </code></pre> <p>the constructor of the model class is called in this method:</p> <pre><code>def setUpDB(test): test.db_engine = create_engine('sqlite:///:memory:', convert_unicode=True) session_factory.configure(bind=test.db_engine) db_session = scoped_session(session_factory) ModelBase.metadata.create_all(test.db_engine) test.client = Client(app, Response) if (hasattr(test, "fixtures")): # load fixtures from json fixtures = test.fixtures if isinstance(fixtures, basestring): fixtures = (fixtures, ) elif isinstance(fixtures, collections.Iterable): pass else: raise Exception( "fixtures attribute needs to be string or iterable of strings") for fixture in fixtures: try: with open(fixture, 'r') as f: fixture = json.loads(f.read()) # apply fixture to database for entry in fixture: model = entry["model"] # import the module containing the Model class model_module = importlib.import_module( model[:model.rfind(".")]) # get the model class model_class = getattr( model_module, model[model.rfind(".") + 1:]) # create an instance of the model class for # each entry in "data" data = entry["data"] for d in data: instance = model_class(**d) instance.save() except IOError as e: print "Could not load Fixture!\n" print e sys.exit(1) </code></pre>
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    1. COInheriting from NoIDModelBase once gets rid of the stack-trace. Apparently you cannot have an AbstractConcreteBase-derived class without child classes floating around?
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