Okay, so, this is convoluted and painful.
We have a project where we use a single table inheritence setup to model
reports. Each report has email addresses to send it to, when to run it,
etc, but is mapped to an individual class for the implementation of what
goes into the report.
Now, to save a *load* (we have a hundred or so reports) of duplicate
lines of code, I wrote classproperty (yeah, before mixins!) so that we
have roughly the following:
class Report(Base):
__tablename__ = 'whatever'
id = Column(Integer, primary_key=True)
python_type = Column('type', String(50))
@classproperty
def __mapper_args__(cls):
if cls.__name__=='Person':
return dict(polymorphic_on=cls.python_type)
else:
return dict(polymorphic_identity=cls.__name__)
class AReport(Report):
# other stuff here
pass
Now, this worked fine in 0.5.8 but
http://www.sqlalchemy.org/trac/ticket/1393 and
http://www.sqlalchemy.org/trac/ticket/1701 put paid to that :-(
The above is a legitimate case for inheriting __mapper_args__, but I
understand the reasons why this is now prohibited.
Okay, so I thought about getting a little creative:
class ComputedMapperArgs:
@classproperty
def __mapper_args__(cls):
if cls.__name__=='Person':
return dict(polymorphic_on=cls.discriminator)
else:
return dict(polymorphic_identity=cls.__name__)
class Person(Base,ComputedMapperArgs):
__tablename__ = 'people'
id = Column(Integer, primary_key=True)
discriminator = Column('type', String(50))
class Engineer(Person,ComputedMapperArgs):
pass
...blows up with:
File
"/mnt/Users/chris.withers/sqlalchemy/lib/sqlalchemy/ext/declarative.py",
line 722, in __init__
_as_declarative(cls, classname, cls.__dict__)
File
"/mnt/Users/chris.withers/sqlalchemy/lib/sqlalchemy/ext/declarative.py",
line 679, in _as_declarative
ignore_nonexistent_tables=True)
File
"/mnt/Users/chris.withers/sqlalchemy/lib/sqlalchemy/sql/util.py", line
205, in join_condition
"between '%s' and '%s'.%s" % (a.description, b.description, hint))
ArgumentError: Can't find any foreign key relationships between 'people'
and 'people'.
As does:
class ComputedMapperArgs:
@classproperty
def __mapper_args__(cls):
return dict(polymorphic_identity=cls.__name__)
class Person(Base):
__tablename__ = 'people'
id = Column(Integer, primary_key=True)
discriminator = Column('type', String(50))
__mapper_args__ = dict(polymorphic_on=discriminator)
class Engineer(Person,ComputedMapperArgs):
pass
...*sigh*.
I'll note that the Mixin recipe also produces this behaviour, even in
0.5.8.
Any ideas? I really don't want to have to go through 100 or so classes
added back in the __mapper_args__ manually...
Chris
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