Setting the FK not performed when relationship is declared

[Nikola Radovanovic]

Hi,

I have a FK in child pointing to parent table. Also, there is a relationship towards parent from children. I would like to create children and set FK to parent explicitly, without previously loading the parent object. After commit, FK of parent in child table is None. In case I remove relationship from child to parent, FK is set properly.

Here is the simplified code:

class Child(Base):

    __tablename__ = 'children'

    id = Column(Integer, primary_key=True)

    parent_id = Column(Integer, ForeignKey('parent.id', ondelete="CASCADE"), nullable=True)

   parent = relationship("Parent", uselist=False) ## <-- when removing this, it works

class Parent(Base):

    __tablename__ = 'parent'

    id = Column(Integer, primary_key=True)

child = Child(parent_id=3)

session.add(child)

session.commit()

# at this moment, child.id is None and not 3., although parent with id ==3 exists in the DB. DB is Postgres 12.

I have tried different loading techniques, but nothing worked.

Can someone please point me into the right direction?

Thank you in advance.

Kindest regards

[Simon King]

I can't see anything obviously wrong with what you've written (you
said "child.id is None", but I assume you meant child.parent_id). Can
you provide a runnable example?

Simon

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[Nikola Radovanovic]

Yes, my bad; it should state: # at this moment, child.parent_id is None. What is strange, when added single_parent=True to the relationship - it then worked as I would expected. I guess its some of my misunderstanding of how relationship works. In case needed, I will set working sample, this one was just extracted from more complicated code.

Thanks!