Hi,
I have been reading the posts on implementing multiple inheritance in
SA. I have a question, for a slightly simpler method of MI, where
there is no overriding of attributes (and e.g. therefore no diamond
problem). This would mean that all underlying tables for classes could
contain columns for all attributes (including the inherited ones,
although these would also be attributes in the subclass). Hmm, ok,
example in code:
from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relation, sessionmaker
Base = declarative_base()
class report(Base):
__tablename__ = "reports"
name = Column(String(50), primary_key=True)
text = Column(String(1000), nullable = False)
def __init__(self, name, text):
self.name = name
self.text = text
def __repr__(self):
return "report: " + self.name + ": " + self.text
class approvable(Base):
__tablename__ = "approvals"
id = Column(Integer, primary_key=True)
approval = Column(Boolean)
def __repr__(self):
return "approved: " + self.approval
class approvable_report(approvable, report):
__tablename__ = "approvable_reports"
def __init__(self, name, text, appr):
self.name = name
self.text = text
self.approval = appr
def __repr__(self):
return report.__repr__(self) + approvable.__repr__(self)
engine = create_engine('sqlite:///:memory:', echo=False)
Base.metadata.create_all(engine)
if __name__ == "__main__":
Session = sessionmaker(bind=engine)
session = Session()
ar = approvable_report("repje", "this is the text of the report",
True)
session.add(ar)
session.commit()
ar2 = session.query(approvable_report).first()
print ar2
It would be fine if the approvable_reports table had all columns of
the superclasses (no overriding == no diamond problem), needing no
foreign keys (or am i missing something), as long as the multiple
inheritance relation in the python classes was maintained. I get the
error:
Traceback (most recent call last):
File "D:\Documents\Code\NetBeans\test\alchemy_test\src\alchemy.py",
line 31, in <module>
class approvable_report(approvable, report):
File "C:\Python27\lib\site-packages\sqlalchemy\ext\declarative.py",
line 1167, in __init__
_as_declarative(cls, classname, cls.__dict__)
File "C:\Python27\lib\site-packages\sqlalchemy\ext\declarative.py",
line 1099, in _as_declarative
ignore_nonexistent_tables=True)
File "C:\Python27\lib\site-packages\sqlalchemy\sql\util.py", line
260, in join_condition
"between '%s' and '%s'.%s" % (a.description, b.description, hint))
sqlalchemy.exc.ArgumentError: Can't find any foreign key relationships
between 'approvals' and 'approvable_reports'.
Is there any way to get this to work? I don't see the need for a
foreign key relationship between sub and superclasses in this case.
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