On Fri, Jan 24, 2020, at 4:25 AM, Maarten De Paepe 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
these will be available if you session.flush(). It should be apparent why they
aren't set ahead of time, if student hasn't been INSERTed yet, it may not even
have a primary key value. Similarly, if the FKs were set immediately, that
doesn't accomodate for if the parent's primary key value were altered (would be
very elaborate to track this at attribute set time) as well as it would be
inefficient if student.teacher were applied multiple times and we really don't
need this FK value until the object is flushed.
>
> 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.
In general, I don't think it should be necessary to know or care about the
foreign key column value ahead of time before SQL is being generated;
similarly, if the concern is that an FK column might be un-set when it's
supposed to be, that's first and foremost what NOT NULL at the schema level is
designed to handle, and otherwise these FK values will be present in the mapper
level hooks before_insert and before_update if you *really* need to get to them
explicitly.
> 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
>
> <https://groups.google.com/d/msgid/sqlalchemy/f8901192-e953-4360-8271-f0b4a6fc68ec%40googlegroups.com?utm_medium=email&utm_source=footer>.
--
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/6a6b3078-6244-414d-8765-646f560e7675%40www.fastmail.com.
