python - How define reverse polymorphic in SQLAlchemy? -

tables

parents:

id  | child_type | child_id --- | ---------- |-------- 1   | student    | 5 7   | student    | 8 9   | employee   | 3 15  | employee   | 8 29  | employee   | 12 

students:

id  | name --- | ---------- 1   | jerry ..  | ... 5   | alex 8   | tom 

employees:

id  | name --- | ---------- 3   | john ..  | ... 8   | mike 12  | susan 

---

files

models.py:

from sqlalchemy.ext.declarative import declarative_base, abstractconcretebase sqlalchemy.schema import column, foreignkey sqlalchemy.sql.sqltypes import integer, string  base = declarative_base()   class parent(base):     __tablename__ = 'parents'     id = column(integer, pirmary_key=true)     child_type = column(string)     child_id = column(integer)     child = relationship('child', uselist=false)   class child(abstractconcretebase, base):     name = column(string)   class student(child):     __tablename__ = 'students'     __mapper_args__ = {         'polymorphic_identity': 'student',         'concrete': true     }     id = column(integer, foreignkey(parent.child_id), primary_key=true)   class employee(child):     __tablename__ = 'employees'     __mapper_args__ = {         'polymorphic_identity': 'employee',         'concrete': true     }     id = column(integer, foreignkey(parent.child_id), primary_key=true) 

main.py:

from sqlalchemy import create_engine sqlalchemy.orm import sessionmaker models import parent  if __name__ == '__main__':     engine = create_engine('mysql~~', encoding='utf-8')     session = sessionmaker(bind=engine)()      parent_success_1 = session.query(parent).get(1)  # <parent(id=1, child_type='student', child_id=5)>     assert parent_success_1.child.name == 'alex'     # success      parent_success_2 = session.query(parent).get(15) # <parent(id=15, child_type='employee', child_id=8)>     assert parent_success_2.child.name == 'mike'     # success      parent_failure = session.query(parent).get(7)    # <parent(id=7, child_type='student', child_id=8)>     assert parent_failure.child.name == 'tom'        # failed     # sqlalchemy.orm.exc.multipleresultsfound: multiple rows found one() 

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parent has child pointer via (child_type, child_id). want access child polymorphic set child_type, child_id.

but when use rails's belongs_to polymorphic style columns, can not load polymorphic instance parent.

if set uselist=true (default) in parent model like:

child = relationship('child') 

parent_failure.child returns:

[<student(id=8, name='tom')>, <employee(id=8, name='mike')>] 

how can set polymorphic_on parent's child_type column using it?