I'm not aware of a built-in way to do this. The SQLAlchemy ORM leans towards you just using the relationships and ignoring the foreign keys.
The FAQ has an entry about this (in one direction at least): https://docs.sqlalchemy.org/en/13/faq/sessions.html#i-set-the-foo-id-attribute-on-my-instance-to-7-but-the-foo-attribute-is-still-none-shouldn-t-it-have-loaded-foo-with-id-7 ...which points to a recipe for loading related objects when the foreign key is set: https://github.com/sqlalchemy/sqlalchemy/wiki/ExpireRelationshipOnFKChange It's probably a similar idea to what you already wrote for introspecting and registering event handlers. Sorry, that's probably not the answer you were hoping for. Is there any chance you could just use relationships and forget about the foreign keys? Simon On Fri, Jan 24, 2020 at 9:25 AM Maarten De Paepe <maar...@adimian.com> wrote: > > Hi, > > I've been looking around a bit, but can't seem to find any info on this. > We use "before flush/commit" events (but this problem applies to validators > as well) to perform a number of things with an istance is persisted. > > One of the issues we're facing a lot is when we create a new instance, and > set a relation, the foreign key counterpart is still None, and the other way > around; If I set the foreign key, the relation is None. Not having the > counterpart set, influences how we write the event/validator, as we have to > load the counterpart on the fly. > > In itself, it's not really an issue, but it's making the code more dense > everytime we have to add the code that loads. > > I was wondering if there was an easy way to keep the foreign key and relation > in sync, when setting one. > > A small piece of code to illustrate the issue > > class Teacher(Base): > __tablename__ = 'teacher' > id = sa.Column(sa.Integer, primary_key=True) > name = sa.Column(sa.String) > > > class Student(Base): > __tablename__ = 'student' > id = sa.Column(sa.Integer, primary_key=True) > name = sa.Column(sa.String) > > teacher_id = sa.Column(sa.Integer, sa.ForeignKey('teacher.id')) > teacher = relationship('Teacher', backref='students') > > > teacher1 = Teacher(name='Mark') > teacher2 = Teacher(name='Eugene') > session.add_all((teacher1, teacher2)) > session.commit() > > student = Student(name='Louis') > student.teacher = teacher1 > # assert student.teacher_id == 1 # Wanted behaviour > > student.teacher_id = 2 > # assert student.teacher == teacher2 # Wanted behaviour > > student.teacher = teacher1 > # assert student.teacher_id == 1 # Wanted behaviour > > I'm able to achieve this behaviour with validators that set it for me, but I > don't want to write a validator everytime I have a fk/relation pair, plus I'm > not sure this behavious is in the scope of validators. > I've managed to write something dynamic that introspects the class and > registers "set" SQLA events, to then set the counterpart, but it's a very > elaborate piece of code. > > So, in conclusion, is there a built in way to keep the fk/relation of an > instance in sync? Or am I going about this the wrong way. > > Thanks for your time! > > Kind regards, > Maarten > > -- > 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 > description. > --- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/f8901192-e953-4360-8271-f0b4a6fc68ec%40googlegroups.com. -- 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 description. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/CAFHwexeONYcGVmwunNwtSnJnt%3D7tp-XBj--UcuNu3w7xqGUH%3DA%40mail.gmail.com.
