Hello world!
I am currently trying to transform my code from classic (schema defined
in an extra python module) to declarative. As intermediate step I am
moving the table definitions to the class implementations.
What I am running into is sketched in the following example. In reality,
I have different modules for Base, Derived and Owner.
-------
from sqlalchemy import *
from sqlalchemy.orm import *
metadata = MetaData()
class Base(object): pass
base_table = Table("base", metadata,
Column("id", Integer, primary_key=True),
Column("target_type", String))
mapper(Base, base_table, polymorphic_on=base_table.c.target_type)
class Derived(Base): pass
derived_table = Table("derived", metadata,
Column("id", Integer, ForeignKey("base.id"), primary_key=True),
Column("owner_id", Integer, ForeignKey("owner.owner_id"), nullable=False))
mapper(Derived, derived_table, polymorphic_identity="derived", inherits=Base)
class Owner(object): pass
owner_table = Table("owner", metadata,
Column("owner_id", Integer, primary_key=True))
mapper(Owner, owner_table)
-------
Running this gives me the following exception:
Traceback (most recent call last):
File "inherit_problem.py", line 21, in <module>
mapper(Derived, derived_table, polymorphic_identity="derived",
inherits=Base)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/orm/__init__.py",
line 890, in mapper
return Mapper(class_, local_table, *args, **params)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/orm/mapper.py",
line 208, in __init__
self._configure_inheritance()
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/orm/mapper.py",
line 259, in _configure_inheritance
self.local_table)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/sql/util.py",
line 227, in join_condition
col = fk.get_referent(left)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/schema.py",
line 1161, in get_referent
return table.corresponding_column(self.column)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/util.py",
line 1517, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File
"/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.6.8dev-py2.6.egg/sqlalchemy/schema.py",
line 1223, in column
"foreign key to target column '%s'" % (self.parent, tname, colname))
sqlalchemy.exc.NoReferencedTableError: Foreign key assocated with column
'derived.owner_id' could not find table 'owner' with which to generate a
foreign key to target column 'owner_id'
The solution is of course to define all the tables first and do the
mapping only after that. This would put me back to point one.
I tried to get around this by patching SA with this patch:
-------
diff -r 27fec7a0e05f lib/sqlalchemy/orm/mapper.py
--- a/lib/sqlalchemy/orm/mapper.py Wed Apr 27 12:51:50 2011 -0400
+++ b/lib/sqlalchemy/orm/mapper.py Thu Apr 28 16:26:57 2011 +0200
@@ -256,7 +256,8 @@
# want (allows test/inheritance.InheritTest4 to pass)
self.inherit_condition = sqlutil.join_condition(
self.inherits.local_table,
- self.local_table)
+ self.local_table,
+
ignore_nonexistent_tables=True)
self.mapped_table = sql.join(
self.inherits.mapped_table,
self.local_table,
-------
This makes the example work, but I'd rather stay with an official
SQLAlchemy. So would it be possible to make this work out of the box? Is
there a better way to implement this?
Any hints are greatly appreciated.
Torsten
PS: I just tried to implement this in a declarative way. This seems to
work fine:
-------
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
DeclBase = declarative_base()
class Base(DeclBase):
__tablename__ = "base"
id = Column(Integer, primary_key=True)
target_type = Column(String)
__mapper_args__ = { "polymorphic_on": target_type }
class Derived(Base):
__tablename__ = "derived"
id = Column(Integer, ForeignKey(Base.id), primary_key=True)
owner_id = Column(Integer, ForeignKey("owner.owner_id"), nullable=False)
owner = relationship("Owner")
__mapper_args__ = { "polymorphic_identity": "derived" }
class Owner(DeclBase):
__tablename__ = "owner"
owner_id = Column(Integer, primary_key=True)
engine = create_engine("sqlite:///:memory:", echo=True)
DeclBase.metadata.create_all(engine)
session = sessionmaker(engine)()
owner = Owner()
derived = Derived(owner=owner)
session.add(derived)
session.commit()
-------
However, I already failed to do the whole conversion in one giant commit,
therefore I would prefer to do it in small steps.
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