定义一个SQLAlchemy模型与其他多个模型之一相关

Question

I'm designing the database for my own Q&A website, which is somewhat similar to Stack Overflow:

1. A "question" can have several "answers", but an "answer" has only one "question".
2. Both "questions" and "answers" can have several "comments", but a "comment" has only one "question" or "answer".

I have no idea how to design such a database. Here is what I've tried:

class Question(db.Model):
    __tablename__ = 'questions'
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.Unicode(64), unique=True)
    body = db.Column(db.UnicodeText)
    author_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    answers = db.relationship('Answer', backref='question', lazy='dynamic')

class Answer(db.Model):
    __tablename__ = 'answers'
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.UnicodeText)
    author_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    question_id = db.Column(db.Integer, db.ForeignKey('questions.id'))

class Comment(db.Model):
    __tablename__ = 'comments'
    id = db.Column(db.Integer, primary_key=True)
    # post_id = db.Column(db.Integer, db.ForeignKey('')) ???

Answer 1

So you've already managed the first point.

What you're looking for is a generic relationship. You can find it in sqlalchemy_utils package.

from sqlalchemy_utils import generic_relationship


class Comment(db.Model):
    __tablename__ = 'comments'
    id = db.Column(db.Integer, primary_key=True)
    object_type = db.Column(db.Unicode(255))
    object_id = db.Column(db.Integer)
    object = generic_relationship(object_type, object_id)

Docs for generic relationship

So basically what it does, it stores the object_type as answer or question and object_id as object's primary key.

Answer 2

I suggest you extract a base class for Question and Answer, eg Post, and make Comment relate to Post, such that a Post can have multiple Comments.

SQLAlchemy ORM supports few strategies to implement inheritance in the database , and the right strategy to choose depends on the way you plan to query your entities. Here's the detailed documentation on how to properly configure it .

So you'd get something like this:

(disclaimer: I have not directly ran this code, but composed it from your example, and my own code. If it ain't working for you, let me know and I'll fix)

class Post(db.Model):
    __tablename__ = 'posts'

    id = db.Column(db.Integer, primary_key=True)
    kind = Column(db.Unicode(64), nullable=False)
    body = db.Column(db.UnicodeText)
    author_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    comments = db.relationship('Comment', backref='post', lazy='dynamic')

    __mapper_args__ = {
        'polymorphic_identity': 'posts',
        'polymorphic_on': kind
    }

class Question(Post):
    __tablename__ = 'questions'

    title = db.Column(db.Unicode(64), unique=True)
    answers = db.relationship('Answer', backref='question', lazy='dynamic')

    __mapper_args__ = {
        'polymorphic_identity': 'questions',
    }

class Answer(Post):
    __tablename__ = 'answers'

    question_id = db.Column(db.Integer, db.ForeignKey('questions.id'))

    __mapper_args__ = {
        'polymorphic_identity': 'answers',
    }

class Comment(db.Model):
    __tablename__ = 'comments'

    id = db.Column(db.Integer, primary_key=True)
    post_id = db.Column(db.Integer, db.ForeignKey('posts.id'))