On Fri, Nov 11, 2016 at 6:59 AM, Alfred Soeng <xiaochang...@gmail.com> wrote:
> It's the issue in the inheritance:
> What I want to do is like this:
> class Employee(Base):
> __tablename__ = 'employee'
> id = Column(
> Integer,
> primary_key=True,
> )
> type_name = Column(DBT.STRING)
> __mapper_args__ = {'polymorphic_on': 'type_name'}
>
>
> class Teacher(Employee):
> __tablename__ = 'teacher'
> id = Column(
> Integer,
> ForeignKey("employee.id"),
> primary_key=True,
> )
> related_student = relationship(
> "Student",
> uselist=False,
> back_populates="teacher")
> __mapper_args__ = {
> 'polymorphic_identity': 'TEACHER',
> 'inherit_condition': (id == Employee.id),
> }
>
>
> class Student(Employee):
> __tablename__ = 'student'
> id = Column(
> Integer,
> ForeignKey("employee.id"),
> primary_key=True,
> )
> teacher_id = Column(Integer, ForeignKey("teacher.id"))
> teacher = relationship(
> "Teacher",
> foreign_keys=[teacher_id],
> back_populates="related_student"
> )
> __mapper_args__ = {
> 'polymorphic_identity': 'STUDENT',
> 'inherit_condition': (id == Employee.id),
> }
>
> If I didn't specify the foreign_keys for Teacher relationship, it will say
> sqlalchemy.exc.AmbiguousForeignKeysError: Could not determine join condition
> between parent/child tables on relationship Teacher.related_student - there
> are multiple foreign key paths linking the tables. Specify the
> 'foreign_keys' argument, providing a list of those columns which should be
> counted as containing a foreign key reference to the parent table.
>
> After I setting the foreign_keys, like below(add foreign_keys for
> related_student relationship):
> class Employee(Base):
> __tablename__ = 'employee'
> id = Column(
> Integer,
> primary_key=True,
> )
> type_name = Column(DBT.STRING)
> __mapper_args__ = {'polymorphic_on': 'type_name'}
>
>
> class Teacher(Employee):
> __tablename__ = 'teacher'
> id = Column(
> Integer,
> ForeignKey("employee.id"),
> primary_key=True,
> )
> related_student = relationship(
> "Student",
> uselist=False,
> foreign_keys=[id]
> back_populates="teacher")
> __mapper_args__ = {
> 'polymorphic_identity': 'TEACHER',
> 'inherit_condition': (id == Employee.id),
> }
>
>
> class Student(Employee):
> __tablename__ = 'student'
> id = Column(
> Integer,
> ForeignKey("employee.id"),
> primary_key=True,
> )
> teacher_id = Column(Integer, ForeignKey("teacher.id"))
> teacher = relationship(
> "Teacher",
> foreign_keys=[teacher_id],
> back_populates="related_student"
> )
> __mapper_args__ = {
> 'polymorphic_identity': 'STUDENT',
> 'inherit_condition': (id == Employee.id),
> }
>
> The error is:
> sqlalchemy.exc.ArgumentError: Student.teacher and back-reference
> Teacher.related_student are both of the same direction symbol('MANYTOONE').
> Did you mean to set remote_side on the many-to-one side ?
>
> Is there any way to solve the problem keeping the inheritance structure and
> 1:1 relationship?
> Thanks so much.
>
Here are your relationship definitions:
teacher = relationship(
"Teacher",
foreign_keys=[teacher_id],
back_populates="related_student"
)
related_student = relationship(
"Student",
uselist=False,
foreign_keys=[id] # <-- this is incorrect
back_populates="teacher")
The "teacher" relationship looks correct - teacher_id is indeed a
foreign key pointing to the Teacher.id column. But the
"related_student" relationship is wrong - Teacher.id is not a foreign
key (for the purposes of this relationship). I'm not sure if you need
the foreign_keys argument here, but if you do, it should probably be
[Student.teacher_id].
Simon
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http://www.sqlalchemy.org/
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