在 Python Flask (sqlalchemy) 中的关系上创建 backref 时出错

Question

我正在创建一个网络应用程序，它可以记录笔记并将其存储在数据库中。我正在使用 MySQL 数据库。

运行代码时，我收到错误。

我认为这个错误是在我的

models.py

这是错误：

sqlalchemy.exc.ArgumentError: Error creating backref 'user_notes' on relationship 'Note.note_author': property of that name exists on mapper 'Mapper[User(users)]'

我有两个文件，这是两个文件：

模型.py

from sqlalchemy import Column, Integer, String, UniqueConstraint, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Note(Base):
    __tablename__ = 'notes'

    id = Column(Integer, primary_key=True, autoincrement=True)
    title = Column(String(100), nullable=False)
    content = Column(String(1000), nullable=False)
    user_id = Column(Integer, ForeignKey('users.id'), nullable=False)
    note_author = relationship('User', backref='user_notes')


    def __repr__(self):
        return f'<Note(id={self.id}, title={self.title}, content={self.content}, user={self.user.username})>'



class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True, autoincrement=True)
    username = Column(String(50), nullable=False, unique=True)
    email = Column(String(100), nullable=False, unique=True)
    password = Column(String(100), nullable=False)

    user_notes = relationship('Note', backref='note_author')

    def __repr__(self):
        return f'<User(id={self.id}, username={self.username}, email={self.email})>'

还有第二个

app.py

from flask import Flask, render_template, request, redirect, url_for
from sqlalchemy import create_engine
from flask_mysqldb import MySQL
from sqlalchemy.orm import sessionmaker
from models import Base, Note, User

# create Flask app
app = Flask(__name__)


app = Flask(__name__)
app.config['MYSQL_HOST'] = 'localhost'
app.config['MYSQL_USER'] = 'root'
app.config['MYSQL_PASSWORD'] = 'password'
app.config['MYSQL_DB'] = 'notes_db'
app.config['MYSQL_CURSORCLASS'] = 'DictCursor'
mysql = MySQL(app)

# create database engine and session
engine = create_engine('mysql+pymysql://user:password@localhost/mydatabase')
Base.metadata.bind = engine
Session = sessionmaker(bind=engine)
session = Session()

@app.route('/')
def index():
    # get all notes
    notes = session.query(Note).all()

    # get note author
    for note in notes:
        note_author = note.note_author
    # get count of total notes
    note_count = session.query(note_author).count()

    # render template with notes and count
    return render_template('index.html', notes=note_author, note_count=note_count)

@app.route('/create-note', methods=['GET', 'POST'])
def create_note():
    if request.method == 'POST':
        # get form data
        title = request.form['title']
        content = request.form['content']
        user_id = int(request.form['user_id'])


        user = session.query(User).get(user_id)

        # create new note
        new_note = Note(title=title, content=content, author=user)
        session.add(new_note)
        session.commit()

        # get user notes
        user_notes = User.user_notes

        # redirect to homepage
        return redirect(url_for('index'))
    else:
        # get all users
        users = session.query(User).all()

        # render form template
        return render_template('create_note.html', users=users)

@app.route('/edit-note/<int:id>', methods=['GET', 'POST'])
def edit_note(id):
    # get note by id
    note = session.query(Note).filter_by(id=id).first()

    if request.method == 'POST':
        # update note
        note.title = request.form['title']
        note.content = request.form['content']
        note.user_id = int(request.form['user_id'])
        session.commit()

        # redirect to homepage
        return redirect(url_for('index'))
    else:
        # get all users
        users = session.query(User).all()

        # render form template with note and users
        return render_template('edit_note.html', note=note, users=users)

@app.route('/delete-note/<int:id>')
def delete_note(id):
    # get note by id
    note = session.query(Note).filter_by(id=id).first()

    # delete note
    session.delete(note)
    session.commit()

    # redirect to homepage
    return redirect(url_for('index'))

if __name__ == '__main__':
    app.run(debug=True)

如何解决我遇到的这个错误？

Answer 1

backref

关键字会自动在相关模型上创建关系，但由于您在两个模型上都创建了关系，因此会引发错误。相反，在两个模型上使用

back_populates

关键字：

class Note(Base):
    note_author = relationship('User', back_populates='user_notes')

class User(Base):
    user_notes = relationship('Note', back_populates='note_author')