Hello everybody.
Is it possible to have the following relationships between three classes
(Animal, Car, Description) ?
* An animal has an attribute description
* A car has an attribute description
* Each description contains his parent as an attribut. So parent can be a
car or an animal
Here is my code :
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
Base = declarative_base()
class Description(Base):
__tablename__ = "descriptions"
id = Column(Integer, primary_key=True)
name = Column(String)
def __init__(self, name):
self.name = name
self.parent = None
def __repr__(self):
return self.name
class Animal(Base):
__tablename__ = "animals"
id = Column(Integer, primary_key=True)
name = Column(String)
id_description = Column(Integer, ForeignKey("descriptions.id"),
nullable = False)
description = relationship("Description", backref = "parent")
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name
class Car(Base):
__tablename__ = "cars"
id = Column(Integer, primary_key=True)
name = Column(String)
id_description = Column(Integer, ForeignKey("descriptions.id"),
nullable = False)
description = relationship("Description", backref = "parent")
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name
engine = create_engine('YOUR DATABASE', echo = True)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
rex = Animal("Rex")
swift = Car("Suzuki Swift")
d1 = Description("Rex is a good dog.")
d2 = Description("What a beautiful car !")
rex.description = d1
d1.parent = rex
swift.description = d2
d2.parent = swift
session.add(rex)
session.add(swift)
session.add(d1)
session.add(d2)
session.commit()
Error :
Traceback (most recent call last):
File "C:\Users\user\Desktop\test.py", line 56, in <module>
rex = Animal("Rex")
File "<string>", line 2, in __init__
File "C:\Python34\lib\site-packages\sqlalchemy\orm\instrumentation.py",
line 347, in _new_state_if_none
state = self._state_constructor(instance, self)
File "C:\Python34\lib\site-packages\sqlalchemy\util\langhelpers.py", line
767, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File "C:\Python34\lib\site-packages\sqlalchemy\orm\instrumentation.py",
line 177, in _state_constructor
self.dispatch.first_init(self, self.class_)
File "C:\Python34\lib\site-packages\sqlalchemy\event\attr.py", line 256,
in __call__
fn(*args, **kw)
File "C:\Python34\lib\site-packages\sqlalchemy\orm\mapper.py", line 3129,
in _event_on_first_init
configure_mappers()
File "C:\Python34\lib\site-packages\sqlalchemy\orm\mapper.py", line 3019,
in configure_mappers
mapper._post_configure_properties()
File "C:\Python34\lib\site-packages\sqlalchemy\orm\mapper.py", line 1810,
in _post_configure_properties
prop.init()
File "C:\Python34\lib\site-packages\sqlalchemy\orm\interfaces.py", line
184, in init
self.do_init()
File "C:\Python34\lib\site-packages\sqlalchemy\orm\relationships.py",
line 1661, in do_init
self._generate_backref()
File "C:\Python34\lib\site-packages\sqlalchemy\orm\relationships.py",
line 1851, in _generate_backref
(backref_key, self, m))
sqlalchemy.exc.ArgumentError: Error creating backref 'parent' on
relationship 'Animal.description': property of that name exists on mapper
'Mapper|Description|descriptions'
It seems impossible to link the classes to the same attribute "parent" in
the class "Description". Is there a solution ?
Thank you for your help !
Olaf
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