Question

在尝试学习Flask时，我正在构建一个简单的Twitter克隆。这将包括用户跟随其他用户的能力。我试图通过SQLAlchemy建立一个关系数据库来允许这个。

我想我需要在用户上自我引用多对多关系。在我到达的SQLAlchemy documentation之后：

#imports omitted    

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///twitclone.db'
db = SQLAlchemy(app)

Base = declarative_base()

user_to_user = Table("user_to_user", Base.metadata,
    Column("follower_id", Integer, ForeignKey("user.id"), primary_key=True),
    Column("followed_id", Integer, ForeignKey("user.id"), primary_key=True)
)

class User(db.Model):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String, unique=False)
    handle = Column(String, unique=True)
    password = Column(String, unique=False)
    children = relationship("tweet")
    following = relationship("user",
                    secondary=user_to_user,
                    primaryjoin=id==user_to_user.c.follower_id,
                    secondaryjoin=id==user_to_user.c.followed_id,
                    backref="followed_by"
    )

#Tweet class goes here

db.create_all()

if __name__ == "__main__":
    app.run()

运行此代码会导致创建数据库时不会显示任何错误消息。然而，简单地省略了将用户连接到用户的整个部分（表）。这是User表的定义：

CREATE TABLE user (
    id INTEGER NOT NULL, 
    name VARCHAR, 
    handle VARCHAR, 
    password VARCHAR, 
    PRIMARY KEY (id), 
    UNIQUE (handle)
)

为什么SQLAlchemy不为用户创建自引用关系？

注意：我是Flask和SQLAlchemy的新手，可能会遗漏一些明显的东西。

Answer 1

好吧，似乎我混淆了两种不同风格的使用SQLAlchemy和Flask：SQLAlchemy的声明性扩展和flask-sqlalchemy扩展。两者的能力相似，不同之处在于烧瓶扩展具有会话处理等一些好处。这就是我重写我的代码以严格使用flask-sqlalchemy的方法。

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy
from datetime import datetime

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///kwek.db'
db = SQLAlchemy(app)

#Table to handle the self-referencing many-to-many relationship for the User class:
#First column holds the user who follows, the second the user who is being followed.
user_to_user = db.Table('user_to_user',
    db.Column("follower_id", db.Integer, db.ForeignKey("user.id"), primary_key=True),
    db.Column("followed_id", db.Integer, db.ForeignKey("user.id"), primary_key=True)
)


class User(db.Model):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=False)
    handle = db.Column(db.String(16), unique=True)
    password = db.Column(db.String, unique=False)
    kweks = db.relationship("Kwek", lazy="dynamic")
    following = db.relationship("User",
                    secondary=user_to_user,
                    primaryjoin=id==user_to_user.c.follower_id,
                    secondaryjoin=id==user_to_user.c.followed_id,
                    backref="followed_by"
    )

    def __repr__(self):
        return '<User %r>' % self.name


class Kwek(db.Model):
    __tablename__ = 'kwek'
    id = db.Column(db.Integer, primary_key=True)
    content = db.Column(db.String(140), unique=False)
    post_date = db.Column(db.DateTime, default=datetime.now())
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

    def __repr__(self):
        return '<Kwek %r>' % self.content

if __name__ == "__main__":
    app.run()

在SQLAlchemy（+ Flask）中创建自引用M2M关系

1 个答案: