[Python] Getting error related to lazy loading when adding many to many relationship

I'm struggling with SQLAlchemy. I have a model with a many to many relationship set up:

class User(Base):
roles: Mapped[List["Role"]] = relationship(
secondary="user_roles", back_populates="users"
)


I'm setting up some code to populate the database with test data:

user = await register_user(
email="me@email.com", username="username", password="test1234"
)
db_session.add(user)
await db_session.commit()

admin_role = Role(name="Admin", owner_id=user.id)
db_session.add(admin_role)

user.roles.append(admin_role)
db_session.add(user)
await db_session.commit()


But when I do this, I get

sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_only() here. Was IO attempted in an unexpected place?


There's a link with that error which suggests that the problem is lazyloading. But in this case, I didn't load anything. I'm confused as to how to establish the cross model relationship.

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