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/
>>>
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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 Example. See http://stackoverflow.com/help/mcve for a full
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--
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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. See http://stackoverflow.com/help/mcve for a full description.
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