SQLAlchemy DELETE: a foreign key contraint fails - sql

Question

I have the following two tables:
class User(Base):
__tablename__ = "user_entity"
id = Column(String(32), primary_key=True, index=True)
email = Column(String(32))
email_constraint = Column(String(32))
email_verified = Column(Boolean)
enabled = Column(Boolean)
federation_link = Column(String(32))
first_name = Column(String(32))
last_name = Column(String(32))
realm_id = Column(String(32))
username = Column(String(32))
created_timestamp = Column(String(32))
service_account_client_link = Column(String(32))
not_before = Column(Integer)
children = relationship("Userattribute", back_populates="parent")
class Userattribute(Base):
__tablename__ = "user_attribute"
name = Column(String(32))
value = Column(String(32))
id = Column(String(32), primary_key=True, index=True)
user_id = Column(String(32), ForeignKey('user_entity.id', ondelete="CASCADE"))
parent = relationship("User", back_populates="children")
I want to delete a user from the user_entity table by his ID:
[SQL: DELETE FROM user_entity WHERE user_entity.id = %s]
Delete implementation:
def delete_user(db: Session, id: str):
return db.query(models.User).filter_by(id=id)
#app.delete("/entity/{id}", response_model=schemas.Userentity)
def del_user(id: str, db: Session = Depends(get_db)):
user = crud.delete_user(db, id=id)
user_obj = user.first()
if user_obj is not None:
user.delete()
db.commit()
return JSONResponse(content={"message": "user erfolgreich gelöscht"})
else:
raise HTTPException(status_code=400, detail=f"ID existiert nicht")
Unfortunately I get the following error:
'Cannot delete or update a parent row: a foreign key constraint fails
Why does this fail? The expected behaviour is that when I delete a user, all user_attributes in the user_attribute table get also deleted. How to fix that?

Answer 1

I can't reproduce your error exactly: user.delete() raises an AttributeError. The correct call would be db.delete(user) I think.
The deletion behaviour must also be specified in the relationship on the parent:
children = relationship(
"Userattribute",
back_populates="parent",
cascade="all, delete"
)
For efficiency, you might also want to specify passive_deletes=True.
See Using foreign key ON DELETE cascade with ORM relationships.