You have missed something important, but I don't know if it will clear
up all your questions :-)
In your example, c.company_id doesn't get populated until the first
flush. Until then, c.company_id is None. So when you wrote:
# Case 1: update the _id doesn't seem to reflect
p.company_id = c.company_id
You were actually writing:
p.company_id = None
This explains why p.company and p.company_id are None all the way
through Case 1.
However, even if you added a session.flush() after adding the objects
to the session, setting a foreign key attribute directly does not
cause the associated relationship to be updated immediately:
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
Hope that helps,
Simon
On Mon, Dec 14, 2020 at 12:50 PM João Miguel Neves
<joao.silva.ne...@gmail.com> wrote:
>
> Hi all,
>
> I'm sorry, as I feel like this is kind of a beginner question. I'd expect
> that when I set a column attribute the respective relationship field would
> change or at least be marked as changed (so a posterior access can be loaded
> later on another access), but I've found a case where it doesn't. I've tried
> to find a bit of code to represent what I'm struggling with, hopefully it
> will be clear enough.
>
> # --- START ---
> from sqlalchemy import Column, create_engine, ForeignKey, Integer, String
>
> from sqlalchemy.orm import sessionmaker, relationship
> from sqlalchemy.ext.declarative import declarative_base
>
>
> engine = create_engine("sqlite://", echo=True)
> Session = sessionmaker(engine)
>
> Base = declarative_base()
>
>
> class Company(Base):
> __tablename__ = "company"
>
> company_id = Column(Integer, primary_key=True)
> name = Column(String(50))
>
>
> class Project(Base):
> __tablename__ = "project"
>
> project_id = Column(Integer, primary_key=True)
> name = Column(String(50))
> company_id = Column(Integer, ForeignKey("company.company_id"), index=True)
> company = relationship("Company")
>
>
> Base.metadata.create_all(engine)
>
> session = Session()
>
> c = Company(name="First Company")
>
> p = Project(name="First Project")
>
> session.add(c)
> session.add(p)
>
> assert p.company is None
> assert p.company_id is None
>
> # Case 1: update the _id doesn't seem to reflect
> p.company_id = c.company_id
>
> assert p.company is None
> assert p.company_id == c.company_id
>
> session.flush()
>
> # Wasn't expecting the None here
> assert p.company is None
> assert p.company_id is None
>
> session.refresh(p)
>
> # Wasn't expecting the None here
> assert p.company is None
> assert p.company_id is None
>
> # Case 2: update the relation works as expected
> p.company = c
>
> assert p.company == c
> assert p.company_id is None
>
> session.flush()
>
> assert p.company == c
> assert p.company_id == c.company_id
>
> session.refresh(p)
>
> assert p.company == c
> assert p.company_id == c.company_id
> # --- END ---
>
> Sorry for bothering, but I'm really puzzled/stuck and feel like I've missed
> something important,
> João
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
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