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
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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
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