How to create a self-referencing one-to-many relationship in Flask-SQLAlchemy? - database

Question

I have a model named Post, which has a boolean field called is_answer. If the is_answer field of a Post is True, it's a "question"; otherwise, it's an "answer". I want to create the following question-answer relationship:
One "question" may have many "answer"s, but one "answer" has and only has one "question". Due to the fact that both "question" and "answer" are essentially Posts, I think the relationship must be self-referencing.
Here is what I've tried:
class Post(db.Model):
__tablename__ = 'posts'
id = db.Column(db.Integer, primary_key=True)
is_question = db.Column(db.Boolean)
post_id = db.Column(db.Integer, db.ForeignKey('posts.id'))
question = db.relationship('Post', backref=db.backref('answer', lazy='dynamic'), uselist=False, lazy='dynamic')
The error is:
ArgumentError: Post.question and back-reference Post.answer are both
of the same direction symbol('ONETOMANY'). Did you mean to set
remote_side on the many-to-one side ?

Answer 1

You need to add remote_side argument to create self-referenced relation. More information in documentaion.
UPDATED: by the way, I think you don't need boolean flag is_question, because you can determine questions and answers by checking post_id field is Null or not.
class Post(Base):
__tablename__ = 'posts'
id = Column(Integer, primary_key=True)
post_id = Column(Integer, ForeignKey('posts.id'))
question = relationship('Post', remote_side=[id], backref=backref('answers'), uselist=False)
Test:
session.add(
Post(
id=1,
post_id=None
)
)
session.add(
Post(
id=2,
post_id=1
)
)
session.add(
Post(
id=3,
post_id=1
)
)
session.commit()
question = session.query(Post).get(1)
print question.answers # output [post2, post3]
answer = session.query(Post).get(2)
print answer.question.id # output 1
# Receive all answers
print session.query(Post).filter(Post.post_id.isnot(None)).all()

Answer 2

You can use the below question and answer table.
class Answer(Base):
__tablename__="answers"
id = Column(Integer, primary_key=True)
mcq_id = Column(Integer,ForeignKey('questions.id'))
answer_text = Column(Text())
is_correct = Column(Boolean, nullable=False, default=False)
class Question(Base):
__tablename__="questions"
id = Column(Integer, primary_key=True)
question_text = Column(Text())
answer_explanation = Column(Text())
answer_choices = relationship('Answer',
primaryjoin="and_(Question.id == Answer.mcq_id )",
cascade="all, delete-orphan",
foreign_keys=[Answer.mcq_id])
# If you have more than one answers then define this function in your model.
def has_more_than_one_correct_answer(self):
count = 0
for choice in self.answer_choices:
if choice.is_correct:
count = count + 1
if count > 1:
return True
else:
return False
You can see the relationship between two tables. And you can access the relationship using joinedload or joinedload_all if you are using sqlalchemy.