[sqlalchemy] Delete fails using automap with joined inheritance
I have been using the automap extension with postgres, with an inheritance
structure using the joined inheritance pattern. I could not figure out a
way to have this reflected from the DB so I define the classes for this
part of my schema explicitly, and when automap initializes, these classes
get used (and modified) for those tables and the rest get autogenerated.
It works fine until I try to delete an instance of an inheriting class:
then I get a circular dependency error which seems to relate to
relationships and backreferences created for the foreign key relationship
underlying the joined inheritance.
The attached code demonstrates the issue. The first run generates the DB
schema from the classes, and works, any number of times. On the second
run, switch the two comments for Base (in two places) to use automap. The
output of the first:
% ./test.py
RELATIONSHIPS: []
Run completed successfully.
% ./test.py
RELATIONSHIPS: [('employee', RelationshipProperty at 0x10263d310;
employee), ('engineer_collection', RelationshipProperty at 0x102663210;
engineer_collection)]
Circular dependency detected. Cycles: {DeleteState(Engineer at
0x1026a9d10)} all edges: {(DeleteState(Engineer at 0x1026a9d10),
DeleteState(Engineer at 0x1026a9d10)),
(ProcessState(OneToManyDP(Employee.engineer_collection), Engineer at
0x1026a9d10, delete=True), DeleteState(Engineer at 0x1026a9d10))}
Notice that the mapper in the first case shows no relationships, despite
the foreign key created for the inheritance. On the other hand when the
same structure is read from the DB by automap, we see forward and back
relationships, which I guess is somehow causing the circular dependency.
Am I doing something wrong in my attempt to use joined inheritance in
conjunction with automap, or is this a bug or something unsupported?
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#!/usr/bin/env python
from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.ext.automap import automap_base
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy.schema import ForeignKey
# SWITCH commenting here and below to test
Base = declarative_base()# Run 1
#Base = automap_base(declarative_base=declarative_base())# Run 2
class Employee(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
__mapper_args__ = {
'polymorphic_identity':'employee', 'polymorphic_on':type
}
class Engineer(Employee):
__tablename__ = 'engineer'
id = Column(Integer, ForeignKey('employee.id'), primary_key=True)
engineer_name = Column(String(30))
__mapper_args__ = {
'polymorphic_identity':'engineer',
}
engine = create_engine(postgresql://user:pw@localhost/test)
Session = sessionmaker(bind=engine)
# SWITCH commenting to test
Base.metadata.create_all(engine)# Run 1
#Base.prepare(engine, reflect=True)# Run 2
# Test
session = Session()
engineer = Engineer(engineer_name='Eng 1', name='Emp 1')
print(RELATIONSHIPS: + str(engineer.__mapper__.relationships.items()))
session.add(engineer)
session.commit()
session = Session()
engineer = session.query(Engineer).all()[0]
session.delete(engineer)
try:
session.commit()
print(Run completed successfully)
except Exception as e:
print(EXCEPTION: + str(e))
Re: [sqlalchemy] Automap and naming of relationship attributes
Well, using the mapper event would be nicer, but in any case I was already
iterating over Base.classes and adding them to my own module's namespace like
so:
globals()[cls.__name__] = cls
It works for the rest of my application being able to see the classes by
importing the module, but apparently not for this. I'm not really expert at
Python class and namespace innards, but from the error message as well as the
default str() output it seems the automap-generated classes considers
themselves to be in the sqlalchemy.ext.automap module but are not registered in
that namespace.
Is there a way to tell the classes to use a different namespace from an
instrument_class handler? (And incidentally I'm already using my own base
class through automap_base(declarative_base(cls=...)) but that doesn't make any
difference.)
On 2014.2.6, at 15:59, Michael Bayer mike...@zzzcomputing.com wrote:
Python pickle can't pickle class instances where the class isn't locatable as
module-level imports. As automap necessarily creates classes on the fly,
these classes aren't part of any module. to have them part of a module you'd
want to use an event to place them in the namespace of one of your own
modules, or you can implement a custom `__reduce__()` method on them (see the
Python docs for __reduce__()).
a good event to use here might be instrument_class:
http://docs.sqlalchemy.org/en/rel_0_9/orm/events.html#sqlalchemy.orm.events.MapperEvents.instrument_class
On Feb 6, 2014, at 4:32 AM, Adrian Robert adrian.b.rob...@gmail.com wrote:
One other point, I was trying out the dogpile cache example and ran into
(after I stuck a .encode('utf-8') into the key mangler since I'm using
Python-3 and pylibmc):
_pickle.PicklingError: Can't pickle class 'sqlalchemy.ext.automap.Person':
attribute lookup sqlalchemy.ext.automap.Person failed
This was fixed by a hack
sqlalchemy.ext.automap.__dict__[cls.__name__] = cls
run over all the automap-created classes. It might be I'm only having to do
this because I'm doing something wrong elsewhere, but I just thought I'd
mention it in case it comes up for someone.
On 2014.2.2, at 14:22, Adrian Robert adrian.b.rob...@gmail.com wrote:
Thanks, that works beautifully.
I had noticed name_for_scalar_relationship parameter but I guess wasn't
confident enough that I understood what was going on to try it. :-[
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Re: [sqlalchemy] Automap and naming of relationship attributes
Thanks, that works beautifully. I had noticed name_for_scalar_relationship parameter but I guess wasn't confident enough that I understood what was going on to try it. :-[ -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.
