Is there a way to create custom classes that inherit from sqlalchemy models in external library?

Angie E

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Jun 3, 2016, 2:44:57 PM6/3/16

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Rather than creating mixin classes that models inherit from, I have a use case that requires me to configure classes the other way around. The classes that would normally be mixin classes need to be the classes that inherit from the models as well as the class that model objects are created from. This is because the models and the mapper configurations are in an external library from the main repository. I need to pass in the host for the engine from the main repository to the models library before any of the models are loaded so they can load with the declarative base already configured. After the engine information is passed in, the session, Base class, and everything is created within a sort of base class that the models inherit from. Here is a simplified example:

class SQLAlchemyBase(object):

metadata = None

Session = None

Base = object

sessionfactory = sessionmaker()

def initialize(self, host):

engine = create_engine(host)

self.metadata = MetaData(bind=engine)

self.Session = scoped_session(self.sessionfactory)

self.Base = declarative_base(metadata=self.metadata)

models = SQLAlchemyBase()

(The models inherit from models.Base)

So the SQLAlchemyBase will be imported into the main repository, the initialize method will be called, passing in the host for the engine, and the models can then be imported. The main repository has its own classes with the same names as the models and have additional methods that a normal mixin class would have to extend functionality. However, I am unable to create model objects using the classes in the main repository because I can't get the mappers to play nice with this unusual inheritance that extends from the external models library. Additionally, in the models library, there are models that have multiple levels of inherited polymorphic relationships. Here is an example that is similar one of the more basic inherited polymorphic relationships:

**Models Library**

class Foo(models.Base):

__tablename__ = "foo"

id = Column(Integer, primary_key=True)

type = Column(String)

foo_bar_id = Column(Integer, ForeignKey("foo_bar.id"))

foo_bar = relationship(Foo, backref=backref("foos"))

__mapper_args__ = {"polymorphic_on": type}

class Bar(Foo):

__mapper_args__ = {"polymorphic_identity": "bar"}

class FooBar(models.Base):

__tablename__ = "foo_bar"

id = Column(Integer, primary_key=True)

**Main Repository**

from separate_library.models import models, Foo as BaseFoo, Bar as BaseBar, FooBar as BaseFooBar

class Foo(BaseFoo):

@classmethod

def custom_create_method(cls, **kw):

foo_obj = cls(**kw)

models.session.add(foo_obj)

models.session.flush()

class Bar(BaseBar):

pass

class FooBar(BaseFooBar):

pass

The original error I was getting was something like this:

"InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Original exception was: Multiple classes found for path "Foo" in the registry of this declarative base. Please use a fully module-qualified path."

So I tried putting the full path in the relationships. Then it started giving me an error like this:

"FlushError: Attempting to flush an item of type <class 'main_module.models.Foo'> as a member of collection "FooBar.foos". Expected an object of type <class 'separate_library.models.Foo'> or a polymorphic subclass of this type. If <class 'main_module.models.Foo'> is a subclass of <class 'separate_library.models.Foo'>, configure mapper "Mapper|Foo|foos" to load this subtype polymorphically, or set enable_typechecks=False to allow any subtype to be accepted for flush.

Essentially, the main problem is getting the classes in the main module to point to and act like the model classes. For example, when I try to create relationships, it says it expected an object of type 'separate_library.models.Foo' instead of 'main_module.models.Foo'. Additionally, in the polymorphic relationships, I can't get the polymorphic_identity to populate for the polymorphic_on column. For example, Bar in the main repository will have the 'type' column empty when the object is initially created.

One idea I tried was to add a metaclass to the declarative base in the models library and modify the mappers in the __init__ method during their initialization. I made progress this way, but haven't gotten it to work completely.

Sorry for the complex explanation, but this is a complex problem. I am not able to change anything about the models or the use case, unfortunately. I have to work within these constraints. If anyone can offer ideas on how to configure the mappers for the classes in the main repository to act like the models in the model library, I would be very grateful.

Mike Bayer

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Jun 3, 2016, 3:38:13 PM6/3/16

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On 06/03/2016 02:44 PM, Angie E wrote:
> Rather than creating mixin classes that models inherit from, I have a
> use case that requires me to configure classes the other way around. The
> classes that would normally be mixin classes need to be the classes that
> inherit from the models as well as the class that model objects are
> created from. This is because the models and the mapper configurations
> are in an external library from the main repository. I need to pass in
> the host for the engine from the main repository to the models library
> before any of the models are loaded so they can load with the
> declarative base already configured.

I hope you are using reflection for these models, otherwise there's no
reason to have a depedency on the Engine for the model declaration.

After the engine information is
> passed in, the session, Base class, and everything is created within a
> sort of base class that the models inherit from. Here is a simplified
> example:
>
>
> class SQLAlchemyBase(object):
> metadata = None
> Session = None
> Base = object
> sessionfactory = sessionmaker()
>
> def initialize(self, host):
> engine = create_engine(host)
> self.metadata = MetaData(bind=engine)
> self.Session = scoped_session(self.sessionfactory)
> self.Base = declarative_base(metadata=self.metadata)
>
> models = SQLAlchemyBase()
>
> (The models inherit from models.Base)
>
>
> So the SQLAlchemyBase will be imported into the main repository, the
> initialize method will be called, passing in the host for the engine,
> and the models can then be imported.

Looks like no reflection taking place. I'd do away with the
MetaData(bind=engine), and just have the engine as part of the
sessionmaker(), and the sessionmaker() here also doesn't need to have
anything to do with the "library", if the "library" is just defining
model classes.

The "bound metadata" pattern is highly discouraged in modern SQLAlchemy
because it leads to this kind of confusion, e.g. that one needs an
Engine in order to declare models. You don't.

that only happens if someone is using a string name inside of
relationship(), like this:

foobars = relationship("Foo")

If the "library" is doing that, and you can't change it, easy solution,
use a different name for your "Foo" that's also in the 3rd party
library. Or declare your classes against a different base:

Base = declarative_base()

class Foo(other_package.Foo, Base):
# ...

class Bar(other_package.Bar, Base):
# ...

I'm not 100% sure that will work but there's probably a way to make that
work.

>
> So I tried putting the full path in the relationships. Then it started
> giving me an error like this:

oh, OK, then you're fine there.

>
> "FlushError: Attempting to flush an item of type <class
> 'main_module.models.Foo'> as a member of collection "FooBar.foos".
> Expected an object of type <class 'separate_library.models.Foo'> or a
> polymorphic subclass of this type. If <class 'main_module.models.Foo'>
> is a subclass of <class 'separate_library.models.Foo'>, configure mapper
> "Mapper|Foo|foos" to load this subtype polymorphically, or set
> enable_typechecks=False to allow any subtype to be accepted for flush.

Well, when this relationship loads, you'd like it to return a list of
main_module.models.Foo objects and not separate_library.models.Foo.
Your model here needs to have a polymorphic identity assigned so that it
can do this. I don't see those set up in your mappings, you need to put
those in explcitly. If you're trying to re-use the same names that the
superclass has, you might need to manipulate
Foo.__mapper__.polymorphic_map directly, not sure.

>
> Essentially, the main problem is getting the classes in the main module
> to point to and act like the model classes. For example, when I try to
> create relationships, it says it expected an object of type
> 'separate_library.models.Foo' instead of 'main_module.models.Foo'.

other than using a different name, there are ways to manipulate the
registry that is doing this but they aren't publicly supported patterns,
unless the easy "use a different Base" trick works out here.

> Additionally, in the polymorphic relationships, I can't get the
> polymorphic_identity to populate for the polymorphic_on column. For
> example, Bar in the main repository will have the 'type' column empty
> when the object is initially created.

when you make a subclass like you are doing, you need to give it its own
identity, so it at least needs to have "polymorphic_identity" set up
locally.

Overall there would be a question here, which is: do you have control
over this external library? If so, I would design it to be used for
subclassing by another declarative system. Assign names like
"AbstractFoo" to the classes and additionally use the __abstract__ =
True declarative flag to mark them as such, then use @declared_attr
around __mapper_args__ to assign mapper arguments like polymorphic
identity to subclasses properly.

If you do *not* have control over this external library, then it is not
intended to be used in this way; the most preferable way to go about
this would be to get the maintainers of it to change it in order to
allow it to be used in this way. An easy way for the external library
to work like this would be that it supplies both the __abstract__
classes with nice names like AbstractFoo, then for its own purposes it
supplies its own Foo / Bar concrete classes on top of a separate
declarative Base, and you'd skip using that part.

>
> One idea I tried was to add a metaclass to the declarative base in the
> models library and modify the mappers in the __init__ method during
> their initialization. I made progress this way, but haven't gotten it to
> work completely.

There's proabably ways to make this work with more tricks and
metaclasses but they aren't supported on the SQLAlchemy end and they
will be brittle, they will break when declarative changes how it does
things internally.

>
> Sorry for the complex explanation, but this is a complex problem. I am
> not able to change anything about the models or the use case,
> unfortunately. I have to work within these constraints.

If these are two different systems within your company, it would be a
better investment to get the upstream system to work more flexibly,
rather than invest efforts working around the system's limitations
downstream. But, if the overlapping names and the polymorphic
identities are the only problem you should be able to work around those.

If anyone can
> offer ideas on how to configure the mappers for the classes in the main
> repository to act like the models in the model library, I would be very
> grateful.
>

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Angie Ellis

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Jun 22, 2016, 4:54:00 PM6/22/16

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Mike,

Thank you for the response! I do have control over the external library. I like your idea having both the abstract classes and concrete classes available in the library because I have use cases for both in my applications. However, I'm not quite sure how to configure the inheritance and declarative base. What am I doing wrong here?

class AbstractFoo(object):

__abstract__ = True

__tablename__ = "foo"

id = Column(Integer, primary_key=True)

type = Column(String)

foo_bar_id = Column(Integer, ForeignKey("foo_bar.id"))

foo_bar = relationship("FooBar", backref=backref("foo"))

__mapper_args__ = {"polymorphic_on": type}

class AbstractBar(object):

__abstract__ = True

__mapper_args__ = {"polymorphic_identity": "bar"}

class AbstractFooBar(object):

__abstract__ = True

__tablename__ = "foo_bar"

id = Column(Integer, primary_key=True)

Base = declarative_base()

class Foo(AbstractFoo, Base):

pass

class Bar(AbstractBar, Foo):

pass

class FooBar(AbstractFooBar, Base):

pass

Does AbstractBar need to inherit from AbstractFoo? Do the abstract classes need to inherit their own declarative base? Do I need to change the inheritance order? The classes Foo, Bar, and FooBar are not getting mapped.

Thanks,

Angie

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Mike Bayer

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Jun 22, 2016, 10:27:46 PM6/22/16

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On 06/22/2016 04:53 PM, Angie Ellis wrote:
> Mike,
>
> Thank you for the response! I do have control over the external library.
> I like your idea having both the abstract classes and concrete classes
> available in the library because I have use cases for both in my
> applications. However, I'm not quite sure how to configure the
> inheritance and declarative base. What am I doing wrong here?
>
>
> class AbstractFoo(object):
> __abstract__ = True
> __tablename__ = "foo"
> id = Column(Integer, primary_key=True)
> type = Column(String)
> foo_bar_id = Column(Integer, ForeignKey("foo_bar.id

> <http://foo_bar.id>"))

> foo_bar = relationship("FooBar", backref=backref("foo"))
>
> __mapper_args__ = {"polymorphic_on": type}
>
>
> class AbstractBar(object):
> __abstract__ = True
> __mapper_args__ = {"polymorphic_identity": "bar"}
>
>
> class AbstractFooBar(object):
> __abstract__ = True
> __tablename__ = "foo_bar"
> id = Column(Integer, primary_key=True)
>
>
>
> Base = declarative_base()
>
>
> class Foo(AbstractFoo, Base):
> pass
>
>
> class Bar(AbstractBar, Foo):
> pass
>
>
> class FooBar(AbstractFooBar, Base):
> pass
>
>
> Does AbstractBar need to inherit from AbstractFoo? Do the abstract
> classes need to inherit their own declarative base? Do I need to change
> the inheritance order? The classes Foo, Bar, and FooBar are not getting
> mapped.

So the way you have it is that those are mixins, which is fine, and I
guess that's how we need it because you want to keep multiple Base's in
play at the same time. You don't need the __abstract__ keyword in that
case. So to map that looks like the following POC:

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.declarative import declared_attr

class AbstractFoo(object):

__tablename__ = "foo"

id = Column(Integer, primary_key=True)
type = Column(String)

@declared_attr
def foo_bar_id(cls):
return Column(Integer, ForeignKey("foo_bar.id"))

@declared_attr
def foo_bar(cls):
return relationship("FooBar", backref=backref("foo"))

__mapper_args__ = {"polymorphic_on": type}

class AbstractBar(AbstractFoo):

__mapper_args__ = {"polymorphic_identity": "bar"}

class AbstractFooBar(object):

__tablename__ = "foo_bar"
id = Column(Integer, primary_key=True)

Base = declarative_base()

class Foo(AbstractFoo, Base):
pass

class Bar(AbstractBar, Foo):
pass

class FooBar(AbstractFooBar, Base):
pass

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)

s.add(Bar(foo_bar=FooBar()))

s.commit()
s.close()

b1 = s.query(Foo).first()
print b1
print b1.foo_bar

>
> Thanks,
> Angie

> <http://foo_bar.id>"))

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Angie Ellis

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Jun 27, 2016, 5:10:06 PM6/27/16

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I implemented the changes you suggested, but I am running into errors with some polymorphic relations. Using a different theme, here is an example:

class AbstractVehicle(object):

__tablename__ = "vehicles"

id = Column(Integer, primary_key=True)

name = Column(String, nullable=False)

type = Column(String)

__mapper_args__ = {"polymorphic_on": type}

class AbstractCar(AbstractVehicle):

__tablename__ = "cars"

id = Column("id", Integer, primary_key=True)

__mapper_args__ = {"polymorphic_identity": "car"}

__table_args__ = (

ForeignKeyConstraint(["id"], ["vehicles.id"]),

)

class AbstractCoupe(AbstractCar):

__mapper_args__ = {"polymorphic_identity": "coupe"}

Base = declarative_base()

class Vehicle(AbstractVehicle, Base):

pass

class Car(AbstractCar, Vehicle):

pass

class Coupe(AbstractCoupe, Car):

pass

This resulted in the following error:

File ".../model/vehicle.py", line 255, in <module>

class Coupe(AbstractCoupe, Car):

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 55, in __init__

_as_declarative(cls, classname, cls.__dict__)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative

_MapperConfig.setup_mapping(cls, classname, dict_)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping

cfg_cls(cls_, classname, dict_)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 131, in __init__

self._setup_table()

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 395, in _setup_table

**table_kw)

File ".../env/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 398, in __new__

"existing Table object." % key)

InvalidRequestError: Table 'cars' is already defined for this MetaData instance. Specify 'extend_existing=True' to redefine options and columns on an existing Table object.

After adding "extend_existing=True" to the AbstractCoupe model, I received this error:

File ".../model/vehicle.py", line 255, in <module>

class Coupe(AbstractCoupe, Car):

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 55, in __init__

_as_declarative(cls, classname, cls.__dict__)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative

_MapperConfig.setup_mapping(cls, classname, dict_)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping

cfg_cls(cls_, classname, dict_)

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 135, in __init__

self._early_mapping()

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 138, in _early_mapping

self.map()

File ".../env/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 530, in map

**self.mapper_args

File "<string>", line 2, in mapper

File ".../env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 627, in __init__

self._configure_properties()

File ".../env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1292, in _configure_properties

self._adapt_inherited_property(key, prop, False)

File ".../env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1514, in _adapt_inherited_property

self._configure_property(key, prop, init=False, setparent=False)

File ".../env/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1541, in _configure_property

prop.columns[0])

File ".../env/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py", line 476, in corresponding_column

if self.c.contains_column(column):

File ".../env/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 754, in __get__

obj.__dict__[self.__name__] = result = self.fget(obj)

File ".../env/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py", line 553, in columns

return self._columns.as_immutable()

AttributeError: 'Table' object has no attribute '_columns'

Is there a way to fix or work around this?

Thanks,

Angie

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Mike Bayer

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Jun 27, 2016, 5:22:02 PM6/27/16

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I think it's OK to add __abstract__ to those mixins, in this case it
seems like the (pretty complicated) class graph is adding up to
something that causes declarative to see AbstractCar as mapped class
thus a "car" table is added. Adding __abstract__ to AbstractCoupe seems
to resolve.

On 06/27/2016 05:10 PM, Angie Ellis wrote:
> I implemented the changes you suggested, but I am running into errors
> with some polymorphic relations. Using a different theme, here is an
> example:
>
>
> class AbstractVehicle(object):
> __tablename__ = "vehicles"
>
> id = Column(Integer, primary_key=True)
> name = Column(String, nullable=False)
> type = Column(String)
>
> __mapper_args__ = {"polymorphic_on": type}
>
>
> class AbstractCar(AbstractVehicle):
> __tablename__ = "cars"
>
> id = Column("id", Integer, primary_key=True)
>
> __mapper_args__ = {"polymorphic_identity": "car"}
> __table_args__ = (

> ForeignKeyConstraint(["id"], ["vehicles.id <http://vehicles.id>"]),

> <http://foo_bar.id>"))

> <mailto:mik...@zzzcomputing.com

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Angie Ellis

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Jul 21, 2016, 7:22:18 PM7/21/16

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Adding __abstract__ = True to the AbstractCoupe did seem to resolve those errors, but it gave me a new one:

________________________________________________________

from sqlalchemy import *

from sqlalchemy.orm import *

from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.ext.declarative import declared_attr

class AbstractVehicle(object):

__tablename__ = "vehicles"

id = Column(Integer, primary_key=True)

name = Column(String, nullable=False)

type = Column(String)

__mapper_args__ = {"polymorphic_on": type}

class AbstractCar(AbstractVehicle):

__tablename__ = "cars"

id = Column("id", Integer, primary_key=True)

__mapper_args__ = {"polymorphic_identity": "car"}

__table_args__ = (

ForeignKeyConstraint(["id"], ["vehicles.id"]),

)

class AbstractCoupe(AbstractCar):

__abstract__ = True

__mapper_args__ = {"polymorphic_identity": "coupe"}

Base = declarative_base()

class Vehicle(AbstractVehicle, Base):

pass

class Car(AbstractCar, Vehicle):

pass

class Coupe(AbstractCoupe, Car):

pass

if __name__ == "__main__":

e = create_engine("sqlite://", echo=True)

Base.metadata.create_all(e)

s = Session(e)

s.add(Coupe(id=10, name="test"))

s.commit()

s.close()

b1 = s.query(Coupe).first()

print(b1)

________________________________________________________

Traceback (most recent call last):

File "test.py", line 53, in <module>

s.add(Coupe(id=10, name="test"))

File "<string>", line 4, in __init__

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/state.py", line 306, in _initialize_instance

manager.dispatch.init_failure(self, args, kwargs)

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__

compat.reraise(exc_type, exc_value, exc_tb)

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 184, in reraise

raise value

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/state.py", line 303, in _initialize_instance

return manager.original_init(*mixed[1:], **kwargs)

File "<string>", line 6, in __init__

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/ext/declarative/base.py", line 649, in _declarative_constructor

setattr(self, k, kwargs[k])

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py", line 223, in __set__

self.impl.set(instance_state(instance),

AttributeError: 'NoneType' object has no attribute 'set'

________________________________________________________

When I take out the arguments and just run:

s.add(Coupe())

It gives me this:

________________________________________________________

Traceback (most recent call last):

File "test.py", line 53, in <module>

s.add(Coupe())

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/session.py", line 1588, in add

self._save_or_update_state(state)

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/session.py", line 1602, in _save_or_update_state

mapper = _state_mapper(state)

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 754, in __get__

obj.__dict__[self.__name__] = result = self.fget(obj)

File "/mnt/hgfs/Documents/git/cc-mysql/env/lib/python3.5/site-packages/sqlalchemy/orm/instrumentation.py", line 116, in mapper

raise exc.UnmappedClassError(self.class_)

sqlalchemy.orm.exc.UnmappedClassError: Class '__main__.Coupe' is not mapped

________________________________________________________

Sorry for the intermittent responses. Thanks so much for your help!

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Mike Bayer

unread,

Jul 22, 2016, 9:36:48 AM7/22/16

to sqlal...@googlegroups.com

the first error is because declarative isn't smart enough to figure out
this many __abstract__ flags, and I'd support efforts to improve this.
But if you pass __abstract__ = False in a few places that gives it the
clues it needs to get to the second case (as well as removing the
__abstract__ = True where not needed).

The second error is more simple, you need to define what table Coupe is
mapped towards; if you want it to be single inheritance off of "cars"
then you need to give it None for __tablename__:

class Vehicle(AbstractVehicle, Base):
pass

class Car(AbstractCar, Vehicle):

__abstract__ = False

class Coupe(AbstractCoupe, Car):
__abstract__ = False
__tablename__ = None

BEGIN (implicit)
2016-07-22 09:35:58,641 INFO sqlalchemy.engine.base.Engine INSERT INTO
vehicles (id, name, type) VALUES (?, ?, ?)
2016-07-22 09:35:58,641 INFO sqlalchemy.engine.base.Engine (10, 'test',
'coupe')
2016-07-22 09:35:58,642 INFO sqlalchemy.engine.base.Engine INSERT INTO
cars (id) VALUES (?)
2016-07-22 09:35:58,642 INFO sqlalchemy.engine.base.Engine (10,)
2016-07-22 09:35:58,642 INFO sqlalchemy.engine.base.Engine COMMIT
2016-07-22 09:35:58,643 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2016-07-22 09:35:58,644 INFO sqlalchemy.engine.base.Engine SELECT
cars.id AS cars_id, vehicles.id AS vehicles_id, vehicles.name AS
vehicles_name, vehicles.type AS vehicles_type
FROM vehicles JOIN cars ON vehicles.id = cars.id
WHERE vehicles.type IN (?)
LIMIT ? OFFSET ?
2016-07-22 09:35:58,644 INFO sqlalchemy.engine.base.Engine ('coupe', 1, 0)
<__main__.Coupe object at 0x7f4a81e5afd0>

On 07/21/2016 07:22 PM, Angie Ellis wrote:
> Adding __abstract__ = True to the AbstractCoupe did seem to resolve
> those errors, but it gave me a new one:
>
> ________________________________________________________
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.ext.declarative import declared_attr
>
>
> class AbstractVehicle(object):
> __tablename__ = "vehicles"
>
> id = Column(Integer, primary_key=True)
> name = Column(String, nullable=False)
> type = Column(String)
>
> __mapper_args__ = {"polymorphic_on": type}
>
>
> class AbstractCar(AbstractVehicle):
> __tablename__ = "cars"
>
> id = Column("id", Integer, primary_key=True)
>
> __mapper_args__ = {"polymorphic_identity": "car"}
> __table_args__ = (

> ForeignKeyConstraint(["id"], ["vehicles.id <http://vehicles.id>"]),
> )
>
>
> class AbstractCoupe(AbstractCar):

> On Mon, Jun 27, 2016 at 2:21 PM, Mike Bayer <mik...@zzzcomputing.com
> <mailto:mik...@zzzcomputing.com>> wrote:
>
> I think it's OK to add __abstract__ to those mixins, in this case it
> seems like the (pretty complicated) class graph is adding up to
> something that causes declarative to see AbstractCar as mapped class
> thus a "car" table is added. Adding __abstract__ to AbstractCoupe
> seems to resolve.
>
>
>
>
> On 06/27/2016 05:10 PM, Angie Ellis wrote:
>
> I implemented the changes you suggested, but I am running into
> errors
> with some polymorphic relations. Using a different theme, here is an
> example:
>
>
> class AbstractVehicle(object):
> __tablename__ = "vehicles"
>
> id = Column(Integer, primary_key=True)
> name = Column(String, nullable=False)
> type = Column(String)
>
> __mapper_args__ = {"polymorphic_on": type}
>
>
> class AbstractCar(AbstractVehicle):
> __tablename__ = "cars"
>
> id = Column("id", Integer, primary_key=True)
>
> __mapper_args__ = {"polymorphic_identity": "car"}
> __table_args__ = (
> ForeignKeyConstraint(["id"], ["vehicles.id

> <http://vehicles.id> <http://vehicles.id>"]),

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