SQLAlchemy如何在flask中查询已登录用户的数据库？

from flask import Flask, request
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = '数据库连接地址'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(50), unique=True)
    password = db.Column(db.String(50))

db.create_all()

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    
    user = User.query.filter_by(username=username, password=password).first()
    
    if user:
        # 用户存在，执行登录成功的操作
        return '登录成功'
    else:
        # 用户不存在或密码错误，执行登录失败的操作
        return '登录失败'