In SQLAlchemy, how should I specify that the relationship field is required? - sqlalchemy

Question

I have a model that depends on some fields on another model. This fields should be present when the record is created, but I do not see a way to enforce that on the database:
class Study(db.Model):
id = db.Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
type = db.Column(Enum(StudyTypeChoices), nullable=False)
owner_id = db.Column(UUID(as_uuid=True), db.ForeignKey('owner.id'), nullable=False)
participants = db.relationship('Participant', lazy=True, cascade='save-update, merge, delete')
How can I make sure that 'participants' is provided when the Study record gets created (similar to what happens with the 'type' field)? I know I can put a wrapper around it to make sure of that, but I am wondering is there is a more neat way of doing it with sqlalchemy.
Edit: This is the definition of the Participant model
class Participant(UserBase):
id = db.Column(UUID(as_uuid=True), db.ForeignKey("user_base.id"), primary_key=True)
study_id = db.Column(UUID(as_uuid=True), db.ForeignKey('study.id'))

Answer 1

You can listen to before_flush events and prevent flushes containing studies without participants by raising an exception for instance.
#event.listens_for(Session, "before_flush")
def before_flush(session, flush_context, instances):
for instance in session.new: # might want to inspect session.dirty as well
if isinstance(instance, Study) and (
instance.participants is None or instance.participants == []
):
raise ValueError(
f"Study {instance} cannot have {instance.participants} participants."
)
This only checks for new studies, you might want to check in session.dirty as well for updated studies.
Full demo:
from sqlalchemy import Column, ForeignKey, Integer, create_engine, event
from sqlalchemy.orm import Session, declarative_base, relationship
Base = declarative_base()
class Study(Base):
__tablename__ = "study"
id = Column(Integer, primary_key=True)
participants = relationship("Participant", uselist=True, back_populates="study")
class Participant(Base):
__tablename__ = "participant"
id = Column(Integer, primary_key=True)
study_id = Column(Integer, ForeignKey("study.id"), nullable=True)
study = relationship("Study", back_populates="participants")
#event.listens_for(Session, "before_flush")
def before_flush(session, flush_context, instances):
for instance in session.new: # might want to inspect session.dirty as well
if isinstance(instance, Study) and (
instance.participants is None or instance.participants == []
):
raise ValueError(
f"Study {instance} cannot have {instance.participants} participants."
)
engine = create_engine("sqlite://", future=True, echo=True)
Base.metadata.create_all(engine)
s1 = Study()
p1_1 = Participant()
p1_2 = Participant()
s1.participants.extend([p1_1, p1_2])
s2 = Study()
with Session(bind=engine) as session:
session.add(s1)
session.commit() # OK
with Session(bind=engine) as session:
session.add(s2)
session.commit() # ValueError