Thanks for pointing this out. It did address this problem already. I just solved this by manually adding all the primary/foreign keys to each classes. Not a big deal, actually it helps clear code!
On Sunday, November 28, 2021 at 6:37:30 PM UTC-8 Mike Bayer wrote: > this is addressed in the docs which discuss "cascading" here: > > > https://docs.sqlalchemy.org/en/14/orm/declarative_mixins.html#mixing-in-columns-in-inheritance-scenarios > > "The declared_attr.cascading feature currently does *not* allow for a > subclass to override the attribute with a different function or value. This > is a current limitation in the mechanics of how @declared_attr is > resolved, and a warning is emitted if this condition is detected. This > limitation does *not* exist for the special attribute names such as > __tablename__, which resolve in a different way internally than that of > declared_attr.cascading." > > > you will see a warning in your program's output: > > SAWarning: Attribute 'record_id' on class <class '__main__.Programmer'> > cannot be processed due to @declared_attr.cascading; skipping > > > as a workaround, you can look for the attribute in __dict__ and return it, > though you still get the warning: > > class has_polymorphic_id(object): > @declared_attr.cascading > def record_id(cls): > if "record_id" in cls.__dict__: > return cls.__dict__['record_id'] > elif has_inherited_table(cls): > return Column(ForeignKey("employee.record_id"), > primary_key=True) > else: > return Column(Integer, primary_key=True) > > > otherwise you'd want to look at "cls" with inspect(cls).local_table etc. > and figure out the correct column to include in the FK if you are doing > things this way. > > > > > > > > On Sun, Nov 28, 2021, at 4:22 PM, niuji...@gmail.com wrote: > > I've just manually put this line to the `Programmer` class definition, but > it still gives me the same error, strangely: > > > class Programmer(Engineer): > __tablename__ = 'programmer' > record_id = Column(ForeignKey('engineer.record_id'), > primary_key=True) > .... > On Sunday, November 28, 2021 at 8:25:30 AM UTC-8 Mike Bayer wrote: > > > > On Sun, Nov 28, 2021, at 4:24 AM, niuji...@gmail.com wrote: > > I'm using the "joined table inheritance" model. I have three levels of > inheritance. > > class has_polymorphic_id(object): > @declared_attr.cascading > def record_id(cls): > if has_inherited_table(cls): > return Column(ForeignKey('employee.record_id'), > primary_key=True) > else: > return Column(Integer, primary_key=True) > > > class Employee(has_polymorphic_id, Base): > __tablename__ = 'employee' > name = Column(String(50)) > type = Column(String(50)) > > __mapper_args__ = { > 'polymorphic_identity':'employee', > 'polymorphic_on':type > } > > class Engineer(Employee): > __tablename__ = 'engineer' > .... > > class Programmer(Engineer): > __tablename__ = 'programmer' > .... > > This only works for the second level, namely `Enginner` can inherits the > foreignkey/primarykey from `Employee`'s mixin, but the next level, the > `Programmer`, python gives me an error: > `sqlalchemy.exc.NoForeignKeysError: Can't find any foreign key > relationships between 'engineer' and 'programmer'.` > > > The "cascading" attribute seems to be working correctly. The error here > is because you aren't providing any column that will allow for a JOIN > between the "programmer" and "engineer" table. > > you would want Programmer.record_id to be a foreign key to > Engineer.record_id, not Employee.record_id. When you load Programmer > rows, the join would be "FROM employee JOIN engineer ON <onclause> JOIN > programmer ON <onclause>". > > > > Is this designed this way? And if I manually set the foreignkey, should > the third level reference to the base level or to its immediate parent > level's primarykey? > > > it has to be to the immediate parent. that's what the error message here > is talking about. > > > > > -- > SQLAlchemy - > The Python SQL Toolkit and Object Relational Mapper > > http://www.sqlalchemy.org/ > > To post example code, please provide an MCVE: Minimal, Complete, and > Verifiable Example. 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