flask做一个登录注册的博客系统代码，带有mysql数据库，数据库通过SQLAlchemy实现

好的，以下是一个简单的使用 Flask 和 SQLAlchemy 的登录注册博客系统代码：

from flask import Flask, render_template, request, redirect, url_for
from flask_sqlalchemy import SQLAlchemy
from werkzeug.security import generate_password_hash, check_password_hash
from datetime import datetime

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql://username:password@localhost/dbname'
app.config['SECRET_KEY'] = 'secretkey'

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(100), unique=True, nullable=False)
    email = db.Column(db.String(100), unique=True, nullable=False)
    password_hash = db.Column(db.String(100), nullable=False)
    posts = db.relationship('Post', backref='author', lazy=True)

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

    def set_password(self, password):
        self.password_hash = generate_password_hash(password)

    def check_password(self, password):
        return check_password_hash(self.password_hash, password)

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    content = db.Column(db.Text, nullable=False)
    date_posted = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)

    def __repr__(self):
        return '<Post %r>' % self.title

@app.route('/')
def home():
    posts = Post.query.order_by(Post.date_posted.desc()).all()
    return render_template('home.html', posts=posts)

@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        username = request.form['username']
        email = request.form['email']
        password = request.form['password']
        confirm_password = request.form['confirm_password']

        if not username or not email or not password or not confirm_password:
            error = 'Please fill out all fields'
            return render_template('register.html', error=error)

        if password != confirm_password:
            error = 'Passwords do not match'
            return render_template('register.html', error=error)

        existing_user = User.query.filter_by(username=username).first()
        if existing_user:
            error = 'Username already exists'
            return render_template('register.html', error=error)

        existing_email = User.query.filter_by(email=email).first()
        if existing_email:
            error = 'Email already registered'
            return render_template('register.html', error=error)

        user = User(username=username, email=email)
        user.set_password(password)
        db.session.add(user)
        db.session.commit()

        return redirect(url_for('login'))

    return render_template('register.html')

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']

        if not username or not password:
            error = 'Please enter a username and password'
            return render_template('login.html', error=error)

        user = User.query.filter_by(username=username).first()

        if not user or not user.check_password(password):
            error = 'Incorrect username or password'
            return render_template('login.html', error=error)

        return redirect(url_for('home'))

    return render_template('login.html')

@app.route('/new_post', methods=['GET', 'POST'])
def new_post():
    if request.method == 'POST':
        title = request.form['title']
        content = request.form['content']

        if not title or not content:
            error = 'Please fill out all fields'
            return render_template('new_post.html', error=error)

        post = Post(title=title, content=content, author=current_user)
        db.session.add(post)
        db.session.commit()

        return redirect(url_for('home'))

    return render_template('new_post.html')

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

注意将 `username`, `password`, 和 `dbname` 替换为你自己的数据库连接信息，以及将 `secretkey` 替换为一个随机的字符串。同时，还需要将模板文件 `home.html`, `login.html`, `register.html`, 和 `new_post.html` 创建并放置在 `templates` 文件夹下，以及静态文件 `style.css` 创建并放置在 `static` 文件夹下。