text: from datetime import datetime
  from itsdangerous import TimedJSONWebSignatureSerializer as Serializer
  from flaskblog import db, login_manager, app
  from flask_login import UserMixin
  
  ### !!! Generate database tables structure first 
  ### !!! by following zreadme_package.txt
  
  @login_manager.user_loader
  def load_user(user_id) :
      return User.query.get(int(user_id))
  
  # defines User table structure in database
  class User(db.Model, UserMixin) :
      # primary_key specifies unique id
      id = db.Column(db.Integer, primary_key=True)
      # max 20 characters, unique, and required
      username = db.Column(db.String(20), unique=True, nullable=False)
      # max 120 characters, unique, and required
      email = db.Column(db.String(120), unique=True, nullable=False)
      # profile image
      image_file = db.Column(db.String(20), nullable=False, default='default.jpg')
      # password (be hashed)
      password = db.Column(db.String(60), nullable=False)
      # This is not a column, but build relationship between post and author
      # 'Post' class being passed
      # backref 'author' is reference to the user by post.author
      posts = db.relationship('Post', backref='author', lazy=True)
  
      # token expires after 1800 seconds (30 min)
      # {'user_id': self.id} as payload
      def get_reset_token(self, expires_sec=1800) :
          s = Serializer(app.config['SECRET_KEY'], expires_sec)
          return s.dumps({'user_id': self.id}).decode('utf-8')
  
      # @staticmethod decorator tells python this member function
      # does not need self as parameter 
      # s.loads(token) = {'user_id': self.id} as payload
      # So, s.loads(token)['user_id'] = self.id
      @staticmethod
      def verify_reset_token(token) :
          s = Serializer(app.config['SECRET_KEY'])
          try:
              user_id = s.loads(token)['user_id']
          except:
              return None
          return User.query.get(user_id)
  
  
      # function to print this class (User)
      def __repr__(self) :
          return f"User('{self.username}','{self.email}','{self.image_file}')"
  
  # defines Post table structure in database
  class Post(db.Model) :
      id = db.Column(db.Integer, primary_key=True)
      title = db.Column(db.String(100), nullable=False)
      # type of DateTime, the current time as default (utcnow)
      # The default is passed as the function name of utcnow. Do not use 
      # utcnow(), which can invoke the function right away. 
      date_posted = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
      content = db.Column(db.Text, nullable=False)
      # point to user who creates this post
      # 'user' in 'user.id' is reference to an user in the User table
      user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
  
      # function to print this class (Post)
      def __repr__(self) :
          return f"Post('{self.title}','{self.date_posted}')"
  