Learn how to build a CRUP app with Flask and SQLAlchemy

A CRUD app is a web app that allows you to create, read, update, and delete things. It is a very common task in web development and very useful for learning how to build web apps.

In this tutorial, you will learn how to build a CRUD application in Flask, and the result will be a working backend for a bookshop web application. We will define services to handle the CRUD operations; GET, POST, PUT, and DELETE requests for the RESTful bookshop API.

Building a bookshop app is helpful for learning because it’s a real example, not a toy project.

This tutorial is based on Flask and Flask-SQLAlchemy extension.

Flask is a microframework for building web apps using Python. It is a very lightweight framework that is easy to learn and use.

Being lightweight does not mean that Flask is not powerful. Whenever you want to use something like ORM (Object Relational Mapping) in your application, you can use extensions that Flask provides. In this tutorial, I’ve used the Flask-SQLAlchemy extension to create a database and a table for storing books.

SQLAlchemy is a Python ORM (Object Relational Mapping) library that makes it easy to work with databases.

Designing the database

Before creating the database, we need to define the database schema and the tables. The schema is the metadata structure of the database while the tables are the actual data that we want to store.

The design of this project is simple: We have a single table called books that stores the books fields: isbn (ISBN number of the book), title, author, and price.

These fields will be stored in the database via the SQLAlchemy ORM. The Flask API will use these fields as the data model for the CRUD operations.

Here is a UML diagram to show the functions used in the API that will depends on the database schema:

Two boxes: Flask and Book. Functions from Flask application (get_books(), get_book(isbn), create_book(), update_book(isbn), and delete_book(isbn) depending on Book model from SQLAlchemy that has the following fields: isbn (as a string), title (as a string), author (as a string), and price (as a float)

Flask app invoked functions that depend on the Book table. (Designed by the Author on Plantuml)

As we can see, Flask API has 5 functions that depend on the Book table. You will see how these functions will invoke appropriate methods from SQLAlchemy. Let’s first see how we structure the API functions. Those functions will be invoked by the Flask API and will be decorated with the @app.route decorator. The mappings for each are shown below:

get_books() to list all the books which maps to /book/list URL using the GET request.
get_book(isbn) to get the specified book defined by the URL parameter isbn that we pass to the function. This function maps to /book/<isbn> URL using the GET request.
create_book() to add a new book to the database. This function maps to /book URL using the POST request.
update_book(isbn) to update the specified book which maps to /book/<isbn> URL using the PUT request.
delete_book(isbn) to delete the specified book which maps to /book/<isbn> URL using the DELETE request.

Structuring the API

To follow along with this project, you can create one file and dump the code in it. You can also create multiple files and import the code from those files to separate the concerns.

I prefer structuring the API code in multiple files. The reason is that it helps you to keep the code organized and also helps you to keep the code clean.

Let’s create a folder called bookshop. Inside that folder, the structure of this project is shown as follows:

.  
├── app  
│   ├── __init__.py  
│   ├── models.py  
│   └── routes.py  
├── bookshop.py  
├── config.py

I tried to make the structure as minimal as possible without using blueprints which I think it would be overkill for this small app. Here is a breakdown of each file:

bookshop.py is the main file that contains the Flask API.
config.py contains the configuration for the API.
app/__init__.py is the file that contains database and application instances.
app/models.py contains the database schema and the ORM.
app/routes.py contains the API functions that will be invoked by the API.

Let’s start with the app/models.py file to create the database.

Installing the dependencies

Before creating the db model, let’s install Flask and Flask-SQLAlchemy extension.

Let’s also install the database engine. In this tutorial, you’ll use MySQL but feel free to use any other database engine. SQLAlchemy supports MySQL, SQLite, Postgres, and more.

For MySQL, install the PyMySQL library.

Let’s install them all:

pip install flask flask-sqlalchemy PyMySQL

It’s time now to create the database.

Creating the database model

Let’s define the database model in the app/models.py file as follows:

from . import db

class Book(db.Model):  
    __tablename__ = 'books'  
    isbn = db.Column(db.Integer, primary_key=True)  
    author = db.Column(db.String(100), nullable=False)  
    title = db.Column(db.String(100), nullable=False)  
    price = db.Column(db.Float)

    def to_json(self):  
        return {  
            'isbn': self.isbn,  
            'author': self.author,  
            'title': self.title,  
            'price': self.price  
        }

In that file, we have defined the table name as books and the fields as:

isbn: the primary key of the table.
author: the author of the book which is a required string field and cannot be NULL. It is restricted to 100 characters long.
title: the title of the book which is a required field and 100 chars long.
price: the price of the book which is a float field and can be NULL.

The to_json() function is used here to convert the Book object to a JSON object that can be returned to the client on the browser. We will see a better way to do that in the next sections.

Note that the Book class is a subclass of the db.Model class. This db instance is defined in the app/__init__.py file as follows:

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

Configuring your Flask app

Now that we have the database model, let’s configure the Flask application for the SQLAlchemy extension.

A good practice for configuring your app is to define a parent config class that will hold common configurations for all environments. Then create an instance of the child config class for your environments. In our case, we will create three environments: development, testing, and production.

Let’s see that in action:

import os

class Config:  
    SQLALCHEMY_TRACK_MODIFICATIONS = False

    @staticmethod  
    def init_app(app):
        pass

class DevelopmentConfig(Config):  
    DEBUG = True  
    SQLALCHEMY_DATABASE_URI = os.getenv("DEV_DATABASE_URL")

class TestingConfig(Config):  
    TESTING = True  
    SQLALCHEMY_DATABASE_URI = os.getenv("TEST_DATABASE_URL")

class ProductionConfig(Config):  
    SQLALCHEMY_DATABASE_URI = os.getenv("DATABASE_URL")

config = {  
    "development": DevelopmentConfig,  
    "testing": TestingConfig,  
    "production": ProductionConfig,  
    "default": DevelopmentConfig  
}

So Config class holds the global configs for the app which are:

SQLALCHEMY_TRACK_MODIFICATIONS set to False to disable the modification tracking system. This is a good practice to avoid the overhead of tracking the changes from Flask-SQLAlchemy to SQLAlchemy library.
init_app() is a static method that is used to initialize the app configurations.

Following this Config parent class, we have three child classes for each environment. Each environment defines the configurations suitable for that environment.

And finally, we have the config dictionary that maps the environment name to the configuration class. The default environment is the development environment which we will use in this tutorial.

The DevelopmentConfig class has the DEBUG attribute set to True because we want to see the debug messages in the browser if there is any error in the API.

Further, it has the SQLALCHEMY_DATABASE_URI attribute set the database URL which is what we define for to connection to the database.

In our case, we set the database URL to the environment variable DEV_DATABASE_URL which is the URL for the MySQL database. Run the following command on your terminal to define that env var:

export DEV_DATABASE_URL=mysql+pymysql://<username>:<password>@localhost:3306/flaskapp

where <username> and <password> are the credentials for the MySQL database and flaskapp is the name of the database. Feel free to replace any of the values with your own.

In case you’re working with another database engine, you can change the DEV_DATABASE_URL to the appropriate URL for that database. For example, if you're using sqlite, you can set it to sqlite:///<path_to_db>.

Now let’s import the config dictionary and start creating the Flask application. Now, the app/__init__.py file looks like the following:

from flask import Flask  
from flask_sqlalchemy import SQLAlchemy  
from config import config

db = SQLAlchemy()

def create_app(config_name):  
    app = Flask(__name__)  
    app.config.from_object(config[config_name])  
    config[config_name].init_app(app)

db.init_app(app)  
    return app

The create_app() function is used to create the app instance based on the environment which is passed as an argument to the function through the config_name parameter.

The app.config.from_object() method is used to load the configuration from the config dictionary. Then that config is used to initialize the app.

Finally, the SQLAlchemy instance db is initialized with the app instance.

Let’s set up the first endpoints for the API which are the GET requests.

Setting up the GET requests

Let’s add the GET request functions to the app/routes.py file:

import os  
from . import create_app  
from .models import Book  
from flask import jsonify

app = create_app(os.getenv('FLASK_CONFIG') or 'default')

@app.route("/book/list", methods=["GET"])  
def get_books():  
    books = Book.query.all()  
    return jsonify([book.to_json() for book in books])

@app.route("/book/<int:isbn>", methods=["GET"])  
def get_book(isbn):  
    book = Book.query.get(isbn)  
    if book is None:  
        abort(404)  
    return jsonify(book.to_json())

The create_app() function instantiates the app instance and then uses the app.route() decorator to register the endpoints. Inside the get_books() method we query the database for all the books using Book.query.all() and then we return the JSON representation of all the books using jsonify() function; it is a helper function that serializes Python objects to JSON.

Now, let’s add some data through SQLAlchemy before we run the app.

Adding data to SQLAlchemy

One way to add data to the database is to open a Flask interactive shell and then create a new instance of the Book model.

Let’s do that by running:

flask shell

This command opens an interactive session for you to execute Python commands. It’s helpful for debugging and testing your code.

We’re now inside the shell. Let’s import the db instance and Book model:

>>> from app import db  
>>> db  
<SQLAlchemy engine=mysql+pymysql://root:***@localhost:3306/flaskapp?charset=utf8>  
>>> from app.models import Book  
>>> Book  
<**class** 'app.models.Book'>

With the assumption that flaskapp database is already created on your machine, let's create the Book table in the database and then define a new book:

>>> db.create_all()  
>>> book = Book(author="Ezz", title="Cleaner Python", price=0.0)  
>>> book  
<app.models.Book **object** at 0x7f404a052e50>  
>>> db.session.add(book)  
>>> db.session.commit()

so now the Book table has been created with db.create_all() which creates all the tables that are subclasses of db.Model.

The book variables is added to the database using db.session.add(). Note that adding the book object to the database does not mean you can query it. It's not yet committed to the database. That's why we need to run db.session.commit() to commit the changes that we made to the database.

Let’s create one more book:

>>> book2 = Book(author="Ahmed", title="Python", price=10.99)  
>>> db.session.add(book2)  
>>> db.session.commit()

So now we have two books in our bookshop. That will suffice to demonstrate listing and playing with the API.

Let’s close the shell with Ctrl+C (or CMD+C) and return to our terminal to run the app.

Running the Flask app

You can run the application by running the following command on your terminal:

export FLASK_APP=bookshop.py  
flask run

The first command defines the FLASK_APP environment variable to point to the bookshop.py file. But you'll find an expected error when you do flask run. That bookshop.py file is empty. Let's fix that and import the app variable from the app/routes.py file:

from app.routes import app

Now, you can run the application and expose the API endpoints at [http://localhost:5000/book/list](http://localhost:5000/book/list.).

In this case, you’ll find the following JSON response:

[  
  {  
    "author": "Ezz",   
    "isbn": 1,   
    "price": 0.0,   
    "title": "Cleaner Python"  
  },   
  {  
    "author": "Ahmed",   
    "isbn": 2,   
    "price": 10.99,   
    "title": "Python"  
  }  
]

And when you invoke this endpoint http://localhost:5000/book/1, you'll get the first book:

{  
  "author": "Ezz",   
  "isbn": 1,   
  "price": 0.0,   
  "title": "Cleaner Python"  
}

Replacing 1 with the ISBN of the book, you'll get the response associated with the book you queried.

Deleting a book

The DELETE request is similar to what we did for the GET request.

Let’s open the app/routes.py file and add the following snippet:

from . import db  
...

@app.route("/book/<int:isbn>", methods=["DELETE"])  
def delete_book(isbn):  
    book = Book.query.get(isbn)  
    if book is None:  
        abort(404)  
    db.session.delete(book)  
    db.session.commit()  
    return jsonify({'result': True})

Here, we use db.session.delete(book) to delete the book from the database and then we commit that change using db.session.commit().

You might be wondering how you test that, especially because the DELETE route is the same as the GET route. To delete a book, you need to use curl and choose the DELETE method because by default it will consider the request as a GET request.

For example, if you want to delete the second book, you can use the following command:

curl http://localhost:5000/book/2 -X DELETE  
{  
  "result": true  
}

which successfully returns a JSON response with the result key set to True as expected.

You can invoke the GET endpoint again to check if the second book is gone through looking at the URL: http://localhost:5000/book/list or by using the curl command:

$ curl http://localhost:5000/book/list  
[  
  {  
    "author": "Ezz",   
    "isbn": 1,   
    "price": 0.0,   
    "title": "Cleaner Python"  
  }  
]

which gives a list of just one book; the first one.

Adding a new book

We can also add a new book to the database by invoking a function using the POST method.

from flask import request  
...

@app.route('/book', methods=['POST'])  
def create_book():  
    if not request.json:  
        abort(400)  
    book = Book(  
        title=request.json.get('title'),  
        author=request.json.get('author'),  
        price=request.json.get('price')  
    )  
    db.session.add(book)  
    db.session.commit()  
    return jsonify(book.to_json()), 201

To test adding a new book, let’s use curl program:

curl -H "Content-Type: application/json" -X POST -d '{"title": "Learning", "author": "Ibrahim", "price": "3.44"}' http://localhost:5000/book  
{  
  "author": "Ibrahim",   
  "isbn": 3,   
  "price": 3.44,   
  "title": "Learning"  
}

Of course, we can add new books using the flask shell as well. The problem with this approach is that you have to import the db and Book instances. To avoid that, now the bookshop.py file would look like the following (after adding a snippet to it):

from app import db  
from app.routes import app  
from app.models import Book

@app.shell_context_processor  
def make_shell_context():  
    return dict(db=db, Book=Book)

The @app.shell_context_processor decorator is used to register a function that will be called to inject variables into the shell session.

The make_shell_context() function returns a dictionary that contains the db and Book instances that we need to add to the shell session so that we can use them in the shell without having to import them.

Updating a book

Updating a book is similar to adding a new one except that we use the PUT method instead of POST.

Let’s add the following to the app/routes.py file:

@app.route('/book/<int:isbn>', methods=['PUT'])  
def update_book(isbn):  
    if not request.json:  
        abort(400)  
    book = Book.query.get(isbn)  
    if book is None:  
        abort(404)  
    book.title = request.json.get('title', book.title)  
    book.author = request.json.get('author', book.author)  
    book.price = request.json.get('price', book.price)  
    db.session.commit()  
    return jsonify(book.to_json())

To test updating a book, let’s use curl again:

curl http://localhost:5000/book/3 -X PUT -H "Content-Type: application/json" -d '{"author": "Ahmed", "title": "Python for Beginners", "price": 12.99}'  
{  
  "author": "Ahmed",   
  "isbn": 3,   
  "price": 12.99,   
  "title": "Python for Beginners"  
}

Conclusion

This tutorial covered the basics of how to create a RESTful API using Flask through a real-world application.

We’ve seen how to use SQLAlchemy to connect to a database, how to create a model, how to map the model to a table, how to create a route, how to use curl program to invoke and test the API, and how to use the flask shell to debug your application.

We also covered how to read, create, update, and delete a book from the CRUD bookshop application.

You can get the complete code for this tutorial from this link.

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Building a CRUD App with Flask and SQLAlchemy

An in-depth tutorial on building the backend of a bookshop CRUD app

Table of contents

Designing the database

Structuring the API

Installing the dependencies

Creating the database model

Configuring your Flask app

Setting up the GET requests

Adding data to SQLAlchemy

Running the Flask app

Deleting a book

Adding a new book

Updating a book

Conclusion

Building a CRUD App with Flask and SQLAlchemy

An in-depth tutorial on building the backend of a bookshop CRUD app

Table of contents

Designing the database

Structuring the API

Installing the dependencies

Creating the database model

Configuring your Flask app

Setting up the GET requests

Adding data to SQLAlchemy

Running the Flask app

Deleting a book

Adding a new book

Updating a book

Conclusion

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