$ pip install -U Flask-SQLAlchemy

from flask import Flask, jsonify, request

from flask_sqlalchemy import SQLAlchemy

​

# Initialize Flask application and SQLAlchemy

app = Flask(__name__)

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db' # Configure the database URI

app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False # disable the feature of flask_sqlalchemy that signals the application everytime a change is about to be made in the database

db = SQLAlchemy(app)

​

class User(db.Model):

    id = db.Column(db.Integer, primary_key=True) # Define the primary key

    username = db.Column(db.String(80), unique=True, nullable=False)

    email = db.Column(db.String(120), unique=True, nullable=False)

    posts = db.relationship('Post', backref='author', lazy=True, cascade="all, delete-orphan")

​

    def __repr__(self):

        return '<User %r>' % self.username

​

class Post(db.Model):

    id = db.Column(db.Integer, primary_key=True)

    title = db.Column(db.String(120), nullable=False)

    content = db.Column(db.String(500), nullable=False)

    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)

​

    def __repr__(self):

        return '<Post %r>' % self.title

​

# Define a route that returns all users in JSON format

@app.route('/users', methods=['GET'])

def get_all_users():

    users = User.query.all()

    return jsonify([e.serialize() for e in users])

​

# Define a route that deletes all users

@app.route('/users', methods=['DELETE'])

def delete_all_users():

    try:

        num_rows_deleted = db.session.query(User).delete()

        db.session.commit()

        return jsonify({"message": "All users deleted successfully", "rows_deleted": num_rows_deleted})

    except:

        db.session.rollback()

        return jsonify({"message": "An error occurred while deleting users"})

​

# Define a route that returns a user by id

@app.route('/users/<int:id>', methods=['GET'])

def get_user_by_id(id):

    user = User.query.get(id)

    if user:

        return jsonify(user.serialize())

    else:

        return jsonify({"message": "User not found"}), 404

​

if __name__ == '__main__':

    db.create_all() # create the table in the database if it does not exist

    app.run(debug=True) # run the application in debug mode

​