On 6/24/14, 5:44 PM, Victor Olex wrote:
> So, what is the right idiom for building SQLAlchemy persistence into
> classes that need to do more than just that i.e. have run-time state.
> I was hoping that deriving from SQLAlchemy model classes, but that
> does not seem to be it. Another option would be to encapsulate a model
> class within the "runtime" class, but that way we need to wrap
> SQLAlchemy session and queries functionality into some helper functions.
Typically the class that has whatever methods and state that you want is
also mapped directly. There's no need to have "User" and "Thinker" as
separate classes.
if you truly don't want anything to do with persistence visibly present
on classes, that's what classical mapping using mapper() and Table was
designed for, or alternatively you can make a business-level class as
abstract, or as a mixin:
class MyClass(Base):
__abstract__ = True
def my_business_method(self):
#...
class MyMappedClass(MyClass):
# ...
depends really on how you need the two roles of business logic and
persistence to be separate. Putting them all together is obviously the
most simplistic but that's what most people do, unless you're trying to
do something more J2EE-ish.
>
> On Friday, May 30, 2014 2:11:53 PM UTC-4, Michael Bayer wrote:
>
> yep… here’s the error:
>
> sqlalchemy.orm.exc.FlushError: Attempting to flush an item of type
> <class '__main__.Thinker'> as a member of collection
> "Address.user". Expected an object of type <class '__main__.User'>
> or a polymorphic subclass of this type. If <class
> '__main__.Thinker'> is a subclass of <class '__main__.User'>,
> configure mapper "Mapper|User|users" to load this subtype
> polymorphically, or set enable_typechecks=False to allow any
> subtype to be accepted for flush.
>
>
> enable_typechecks=False disables this check:
>
> user = relationship("User", enable_typechecks=False,
> backref=backref('addresses', order_by=id))
>
> it just means that later on, when you hit some_address.user, you
> may get a User back, not a Thinker (or you will, if it hasn’t been
> expired. you can’t rely on it being consistent). If that’s OK,
> then set the flag - it just wants to check that this is what you
> intend.
>
>
>
>
> On May 30, 2014, at 1:51 PM, Victor Olex
> <[email protected] <javascript:>> wrote:
>
>> Hello all, long time no see...
>>
>> Is it OK to create classes, which inherit from mapped classes,
>> but are not meant to be persited and how to do it as to avoid
>> FlushError on related classes?
>>
>> from sqlalchemy import *
>> from sqlalchemy.ext.declarative import declarative_base
>> from sqlalchemy.orm import Session, relationship, backref
>>
>> Base = declarative_base()
>>
>> class User(Base):
>> __tablename__ = 'users'
>> id = Column(Integer, Sequence('user_id_seq'), primary_key=True)
>> name = Column(String(50))
>> fullname = Column(String(50))
>> password = Column(String(12))
>>
>> class Address(Base):
>> __tablename__ = 'addresses'
>> id = Column(Integer, primary_key=True)
>> email_address = Column(String, nullable=False)
>> user_id = Column(Integer, ForeignKey('users.id
>> <http://users.id>'))
>> user = relationship("User", backref=backref('addresses',
>> order_by=id))
>>
>> class Thinker(User):
>> thought = 'Thoughts are not to be persited'
>>
>> e = create_engine('sqlite:///', echo=True)
>> Base.metadata.bind = e
>> Base.metadata.create_all()
>>
>> t = Thinker(name='Descartes')
>> s = Session(bind=e)
>> s.add(t)
>> s.commit() # no problem
>> a = Address(user=t, email='[email protected] <javascript:>')
>> a = Address(user=t, [email protected] <javascript:>')
>> s.commit() # FlushError
>>
>> Thanks,
>>
>> V.
>>
>>
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