DJANGO APPS AND ORMPRODUCT VALLEY MEETUP GROUP BANGALORE (A PRODEERS CHAPTER)

- BY UDIT GANGWANI

ABOUT ME

I WEAR MULTIPLE HATSSOFTWARE DEVELOPER

PRODUCT MANAGERENTREPRENEUR

I LOVE TO TRAVELMY FRIENDS CALL ME PIZZA MANIAC

WHAT WILL WE LEARN TODAY

INTRODUCTION TO DJANGO

HOW TO GET STARTED WITH DJANGO

COMPONENTS OF DJANGO FRAMEWORK

DJANGO MODELS, MODEL MANAGERS AND QUERY SETS

HOW DO DJANGO MODELS WORK

WHAT IS DJANGO ?

Django is a high-level Python Web framework that encourages rapid

development and clean, pragmatic design. Built by experienced developers, it

takes care of much of the hassle of Web development, so you can focus on

writing your app without needing to reinvent the wheel.

-- https://www.djangoproject.com/

WHY DJANGO ?

• Incredible programming language (Python)

• Perfect ORM for handling database

• Offers great control (Explicit is better than implicit)

• Django Admin GUI

• Simple and smaller footprint to get started

• Very big & active community

• Lots of Django packages for almost every functionality

• Support for large variants of Databases

• Django REST Framework

MTV ARCHITECTUREDjango follows MTV architecture:

M – Model • app/models.py

V – View• app/views.py

T – Templates• app/templates/*.html

It is analogous to MVC architecture.

View in MVC corresponds to Template in MTV

Controller in MVC corresponds to View in MTV

POPULAR SITES BUILT WITH DJANGO

• Pinterest

• Instagram

• Discus

• Spotify

• Washington Post

• Firefox

• NASA

• BitBucket

• Prezi

• EventBrite

DJANGO SETUP AND A DEMO

ENVIRONMENT SETUP

• Standard Environment

• Common environment and set of packages for each project on your system

• Virtual Environment

• Isolated environment for each Django project

• No limits on the number of environments

• Can have different python version for each project

• Using virtualenv or pyenv

STEPS TO SETUP VIRTUAL ENVIRONMENT

• pip install virtualenv

• virtualenv .\app

• source \app\bin\activate

GETTING STARTED WITH DJANGO

• Install Django using pip

• Create a project using django-admin

• Create your database

• Configure DB settings in settings.py

• Define your models

• Add external modules

• Write your Templates

• Define your Urls

• Write your Views and bind them to Urls

• Test application

• Deploy application using NginX or Apache

DJANGO PROJECT STRUCTURE

• Each Django app is a Python Package

• Each app contains one or more Python modules (files of python code)

• To form a package every app directory must contain an __init__.py file

BLOG APPLICATION DEMO

DJANGO COMPONENTS

URL’S DISPATCHER/ROUTING

• Django determines the root URLconf module to use• Django loads the Python module and looks for the variable urlpatterns• Django runs through each pattern and stops at the one which matches • Once the regex matches, Django calls the given View

VIEWS

• A Django view is a callable which takes a request and returns a response

MODELS & MODEL FIELDS

• Each model maps to a single database table.

• Each model is a Python class that subclasses django.db.models.Model

• Each attribute of the model represents a database field

TEMPLATES

• A template contains the static parts of the desired HTML output as well as

some special syntax describing how dynamic content will be inserted

MIDDLEWARES

• Middleware is a framework of hooks into Django’s request/response

processing. It’s a light, low-level “plugin” system for globally altering

Django’s input or output.

DJANGO MODELS

MIGRATIONS

• Migrations are Django’s way of propagating changes you make to your

models (adding a field, deleting a model, etc.) into your database schema.

• They’re designed to be mostly automatic

There are several commands which you will use to interact with migrations:•migrate, which is responsible for applying migrations, as well as unapplying and listing their status.•makemigrations, which is responsible for creating new migrations based on the changes you have made to your models.•sqlmigrate, which displays the SQL statements for a migration.•showmigrations, which lists a project’s migrations.

MODEL RELATIONSHIPS

Many to One Relationships Many to Many Relationships

One to One Relationships

MODEL QUERIES

• Creating Objects

• Retrieving Objects

• Create query set objects using your model manager. A query set represents the

collection of objects from database

• Query sets are lazy

• Retrieving specific objects using filters

MODEL QUERIES ON RELATED OBJECTS

One to Many Field

One to One Many Reverse lookup

Many to Many Field

One to One Field

MANAGERS

• A Manager is the interface through which database query operations are

provided to Django models. At least one Manager exists for every model in a

Django application.

• Adding extra Manager methods is the preferred way to add “table-level”

functionality to your models. (For “row-level” functionality – i.e., functions

that act on a single instance of a model object – use Model methods, not

custom Manager methods.)

MANAGERS EXAMPLE

QUERY SETS

• Internally, a QuerySet can be constructed, filtered, sliced, and generally

passed around without actually hitting the database. No database activity

actually occurs until you do something to evaluate the queryset.

• Important methods that return new Query set

• filter(), exclude(), annotate(), order_by(), reverse(), distinct(), values(), all()

• Important methods that do not return Query set

• get(), create(), update(), aggregate(), iterator(), count()

Q OBJECTS

• A Q object (django.db.models.Q) is an object used to encapsulate a

collection of keyword arguments. It is used to execute more complex queries

(for example, queries with OR statements)

AGGREGATION

Aggregation for entire table

Aggregation for each item in table

HOW DO DJANGO MODELS WORK ?

HOW DO MODELS WORK - METACLASSES

Lets understand what goes on from the time a model is imported until an

instance is created by the user.

Lets look at an example model

HOW DO MODELS WORK - METACLASSES

So ModelBase.__new__ is called to create this new Example class. It is important to realise that we are creating the class object here, not an instance of it

The Model class (see base.py) has a __metaclass__ attribute that defines ModelBase (also in base.py) as the class to use for creating new classes.

The __new__ method is required to return a class object that can then be instantiated (by calling Example() in our case).

A new class object with the Example name is created in the right module namespace. A _meta attribute is added to hold all of the field validation, retrieval and saving machinery

HOW DO MODELS WORK - METACLASSES

Each attribute is then added to the new class object. Putting this in the context of our example, the static and __unicode__ attributes would be added normally to the class as a string object and unbound method, respectively.

In case of Field, it does not add the new attribute to the class we are creating (Example). Instead it adds itself to the Example._meta class, ending up in the Example._meta.fields list

The ModelBase._prepare method is called. This sets up a few model methods that might be required depending on other options you have selected and adds a primary key field if one has not been explicitly declared.

The registration is done right at the end of the ModelBase.__new__ method. The register_models() function in loaders.py

REFERENCES

• Django Orm at Django under the hood: https://www.youtube.com/watch?v=CGF-0csOjPw

• Django Topics: https://docs.djangoproject.com/en/1.9/topics/

• What is a MetaClass in Python: http://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python

• How do Django Models work:

• http://stackoverflow.com/questions/12006267/how-do-django-models-work

• https://code.djangoproject.com/wiki/DevModelCreation

• Django Cookbook: https://code.djangoproject.com/wiki/CookBook

• Python3 Cookbook: http://python3-cookbook.readthedocs.io/zh_CN/latest/c09/p18_define_classes_programmatically.html

REFERENCES

• Try Django Blog Project:

• https://github.com/codingforentrepreneurs/try-django-19

• https://www.codingforentrepreneurs.com/projects/try-django-19/

• Django Models and Migrations:

• http://www.webforefront.com/django/setupdjangomodels.html

• Making Django Queries :

• https://docs.djangoproject.com/en/1.9/topics/db/queries/

• Understanding Django Model Managers and QuerySets:

• https://docs.djangoproject.com/en/1.9/ref/models/querysets/

• https://docs.djangoproject.com/en/1.9/topics/db/managers/

THANK YOU