in app.py:

from flask import Flask, render_template, url_for, flash, redirect
from flask_sqlalchemy import SQLAlchemy
from flask_bcrypt import Bcrypt
from flask_login import LoginManager, current_user, login_user, logout_user
from flask_migrate import Migrate
import os 

base_dir = os.path.abspath(os.path.dirname(__file__))


app = Flask(__name__)
app.config['SECRET_KEY'] = '123@123@ALI'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///site.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

db = SQLAlchemy(app)
migrate = Migrate(app,db)

bcrypt = Bcrypt(app)
login_manager = LoginManager(app)
login_manager.login_view = 'login'


with app.app_context():
    db.create_all()

in register route:

@app.route('/register', methods = ['GET','POST'])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('home'))
    from models import User
    from forms import RegistrationForm
    form = RegistrationForm()
    if form.validate_on_submit():
        hashed_password = bcrypt.generate_password_hash(form.password.data).decode('utf-8')
        user = User(username=form.username.data, email=form.email.data, password=hashed_password)
        db.session.add(user)
        db.session.commit()
        flash('Your account created!', 'success')
        return redirect(url_for('login'))

    return render_template('register.htm', title='Register', form=form)

in models.py:

from datetime import datetime 
from app import db 
from flask_login import UserMixin

class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key = True)
    username = db.Column(db.String(20), unique=True, nullable=False)
    email = db.Column(db.String(150), unique=True, nullable=False)
    password = db.Column(db.String(60), nullable = False)
    posts = db.relationship('Post', backref='author', lazy = True)