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Mastering Flask
Mastering Flask

Mastering Flask: Gain expertise in Flask to create dynamic and powerful web applications

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Mastering Flask

Chapter 2. Creating Models with SQLAlchemy

As previously stated, models are a means of abstracting and giving a common interface to data. In most web applications, data is stored and retrieved from a Relational Database Management System (RDBMS), which is a database that holds data in a tabular format with rows and columns and is able to compare data across tables. Some examples include MySQL, Postgres, Oracle, and MSSQL.

In order to create models on top of our database, we will use a Python package named SQLAlchemy. SQLAlchemy is a database API at its lowest level and performs Object Relational Mapping (ORM) at its highest level. An ORM is a technique to pass and convert data between two sources with different types of systems and data structures. In this case, it converts data between the large amount of types in databases versus the mix of types and objects in Python. Also, a programming language such as Python allows you to have different objects that hold references to each...

Setting up SQLAlchemy

In order to follow along in this chapter, you will need a running database if you do not already have one. If you have never installed a database or you do not have a preference, SQLite is the best option for beginners.

SQLite is a SQL that is fast, works without a server, and is entirely contained in one file. Also, SQLite is natively supported in python. If you choose to go with SQLite, a SQLite database will be created for you in the Our first model section.

Python packages

To install Flask SQLAlchemy with pip, run the following:

$ pip install flask-sqlalchemy

We will also need to install specific packages for the database you chose to use that will act as the connector for SQLAlchemy. SQLite users can skip this step:

# MySQL
$ pip install PyMySQL
# Postgres
$ pip install psycopg2
# MSSQL
$ pip install pyodbc
# Oracle
$ pip install cx_Oracle

Flask SQLAlchemy

Before we can abstract our data, we need to set up Flask SQLAlchemy. SQLAlchemy creates its database connection...

Our first model

You may have noted that we did not actually create any tables in our database to abstract off of. This is because SQLAlchemy allows us to create either models from tables or tables from our models. This will be covered after we create the first model.

In our main.py file, SQLAlchemy must first be initialized with our app as follows:

from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config.from_object(DevConfig)
db = SQLAlchemy(app)

SQLAlchemy will read our app's configuration and automatically connect to our database. Let's create a User model to interact with a user table in the main.py file:

class User(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    username = db.Column(db.String(255))
    password = db.Column(db.String(255))

    def __init__(self, username):
        self.username = username

    def __repr__(self):
        return "<User '{}'>".format(self.username)

What have we accomplished? We now...

CRUD

In every storage mechanism for data, there are four basic types of functions: Create, Read, Update, and Delete (CRUD). These allow all the basic ways of manipulating and viewing data needed for our web apps. To use these functions, we will use an object on the database named the session. Sessions will be explained later in the chapter, but for now, think of them as a storage location for all of our changes to the database.

Creating models

To create a new row in your database using our models, add the model to the session and commit objects. Adding an object to the session marks its changes for saving, and committing is when the session is saved to the database as follows:

>>> user = User(username='fake_name')
>>> db.session.add(user)
>>> db.session.commit()

It is simple to add a new row to our table.

Reading models

After we have added data to our database, data can be queried using Model.query. For those who use SQLAlchemy, this is shorthand for db.session...

Relationships between models

Relationships between models in SQLAlchemy are links between two or more models that allow models to reference each other automatically. This allows naturally related data, such as comments to posts, to be easily retrieved from the database with its related data. This is where the R in RDBMS comes from, and it gives this type of database a large amount of power.

Let's create our first relation. Our blogging website is going to need some blog posts. Each blog post is going to be written by one user, so it makes sense to link posts back to the user that wrote them to easily get all posts by a user. This is an example of a one-to-many relationship.

One-to-many

Let's add a model to represent blog posts on our website:

class Post(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    title = db.Column(db.String(255))
    text = db.Column(db.Text())
    publish_date = db.Column(db.DateTime())
    user_id = db.Column(db.Integer(), db.ForeignKey(&apos...

The convenience of SQLAlchemy sessions

Now that you understand the power of SQLAlchemy, you can also understand what the SQLAlchemy session object is and why web apps should never be made without them. As stated before, the session can be simply described as an object that tracks the changes in our models and commits them to the database when we tell it to. However, there is a bit more to it than this.

First, the session is the handler for transactions. Transactions are sets of changes that are flushed to the database on commit. Transactions provide a lot of hidden functionality. For example, transactions automatically determine which objects will be saved first when objects have relations. You might have noted this when we were saving tags in the previous section. When we added tags to the posts, the session automatically knew to save the tags first despite the fact that we did not add it to be committed. If we are working with raw SQL queries and a database connection, we would have to...

Setting up SQLAlchemy


In order to follow along in this chapter, you will need a running database if you do not already have one. If you have never installed a database or you do not have a preference, SQLite is the best option for beginners.

SQLite is a SQL that is fast, works without a server, and is entirely contained in one file. Also, SQLite is natively supported in python. If you choose to go with SQLite, a SQLite database will be created for you in the Our first model section.

Python packages

To install Flask SQLAlchemy with pip, run the following:

$ pip install flask-sqlalchemy

We will also need to install specific packages for the database you chose to use that will act as the connector for SQLAlchemy. SQLite users can skip this step:

# MySQL
$ pip install PyMySQL
# Postgres
$ pip install psycopg2
# MSSQL
$ pip install pyodbc
# Oracle
$ pip install cx_Oracle

Flask SQLAlchemy

Before we can abstract our data, we need to set up Flask SQLAlchemy. SQLAlchemy creates its database connection...

Our first model


You may have noted that we did not actually create any tables in our database to abstract off of. This is because SQLAlchemy allows us to create either models from tables or tables from our models. This will be covered after we create the first model.

In our main.py file, SQLAlchemy must first be initialized with our app as follows:

from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config.from_object(DevConfig)
db = SQLAlchemy(app)

SQLAlchemy will read our app's configuration and automatically connect to our database. Let's create a User model to interact with a user table in the main.py file:

class User(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    username = db.Column(db.String(255))
    password = db.Column(db.String(255))

    def __init__(self, username):
        self.username = username

    def __repr__(self):
        return "<User '{}'>".format(self.username)

What have we accomplished? We now have a model that is based on...

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Key benefits

  • • Work with scalable Flask application structures to create complex web apps
  • • Discover the most powerful Flask extensions and learn how to create one
  • • Deploy your application to real-world platforms using this step-by-step guide
  • s

Description

Starting from a simple Flask app, this book will walk through advanced topics while providing practical examples of the lessons learned. After building a simple Flask app, a proper app structure is demonstrated by transforming the app to use a Model-View-Controller (MVC) architecture. With a scalable structure in hand, the next chapters use Flask extensions to provide extra functionality to the app, including user login and registration, NoSQL querying, a REST API, an admin interface, and more. Next, you’ll discover how to use unit testing to take the guesswork away from making sure the code is performing as it should. The book closes with a discussion of the different platforms that are available to deploy a Flask app on, the pros and cons of each one, and how to deploy on each one.

Who is this book for?

If you are a Flask user who knows the basics of the library and how to create basic web pages with HTML and CSS, and you want to take your applications to the next level, this is the book for you. Harnessing the full power of Flask will allow you to create complex web applications with ease.

What you will learn

  • • Set up a best practices Python environment
  • • Use SQLAlchemy to programmatically query a database
  • • Develop templates in Jinja
  • • Set up an MVC environment for Flask
  • • Discover NoSQL, when to use it, when not to, and how to use it
  • • Develop a custom Flask extension
  • • Use Celery to create asynchronous tasks
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Publication date : Sep 30, 2015
Length: 288 pages
Edition : 1st
Language : English
ISBN-13 : 9781784393656
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Publication date : Sep 30, 2015
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Edition : 1st
Language : English
ISBN-13 : 9781784393656
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Table of Contents

14 Chapters
1. Getting Started Chevron down icon Chevron up icon
2. Creating Models with SQLAlchemy Chevron down icon Chevron up icon
3. Creating Views with Templates Chevron down icon Chevron up icon
4. Creating Controllers with Blueprints Chevron down icon Chevron up icon
5. Advanced Application Structure Chevron down icon Chevron up icon
6. Securing Your App Chevron down icon Chevron up icon
7. Using NoSQL with Flask Chevron down icon Chevron up icon
8. Building RESTful APIs Chevron down icon Chevron up icon
9. Creating Asynchronous Tasks with Celery Chevron down icon Chevron up icon
10. Useful Flask Extensions Chevron down icon Chevron up icon
11. Building Your Own Extension Chevron down icon Chevron up icon
12. Testing Flask Apps Chevron down icon Chevron up icon
13. Deploying Flask Apps Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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Full star icon Full star icon Full star icon Half star icon Empty star icon 3.6
(25 Ratings)
5 star 32%
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2 star 12%
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Gerald Brown Oct 16, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This is a book for Python programmers that want to include Flask in their applications. It covers everything from start to finish. The start it shows how to obtain and install Flask and at the end it tells how to deploy the application on your own server, on Heroku and on Amazon web services. In between it tells about databases, explaining the difference between RDBMS and NoSQL databases and which should be used depending on the application. It talks about SQLAlchemy, which is used for connecting to the database. It also presents information on using Jinja to create the user views. It lists a few of the more popular Flask extensions and how to create your own.All in all I would recommend this book.
Amazon Verified review Amazon
Daniel Flehner Heen Dec 16, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Really nice balance between to the point information and touching subjects that widen the horizon.Easy to follow and fun to read.You should be able to create great apps/sites with the help of this book
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Shashank Naik Dec 26, 2023
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Nigmatov Sardor Oct 14, 2023
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very good lesson
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Jim Olivi Nov 20, 2022
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