Session Overview

QuerySets Introduction

A QuerySet is a collection of records retrieved from a database table.

Django uses QuerySets to read data from database tables.

Example

Category.objects.all()

The above statement retrieves all records from the Category table.

Category Table
       ↓
QuerySet
       ↓
Python Object
       ↓
Template

Fetching Data from Database

We can retrieve records from the Category table using QuerySets.

File Location

demo/views.py

Import Category Model

from .models import Category

Fetch Records

from django.shortcuts import render

from .models import Category

def categories(request):

    categories = Category.objects.all()

    return render(
        request,
        'categories.html'
    )

Passing Data to Template

After retrieving records from the database, we must send them to the Template.

Update View

from django.shortcuts import render

from .models import Category

def categories(request):

    categories = Category.objects.all()

    return render(
        request,
        'categories.html',
        {
            'categories': categories
        }
    )

Understanding the Code

Displaying Records Using for Loop

Django Templates use the for tag to display multiple records.

File Location

demo/templates/categories.html

Display Records

{% extends 'base.html' %}

{% block content %}

<h2>Categories</h2>

<ul>

    {% for item in categories %}

        <li>{{ item.Name }}</li>

    {% endfor %}

</ul>

{% endblock %}

Output

Technology

Travel

Food

Empty Data Handling

Sometimes a database table may not contain any records.

Instead of displaying a blank page, we should show a meaningful message.

Example

{% extends 'base.html' %}

{% block content %}

<h2>Categories</h2>

{% if categories %}

    <ul>

        {% for item in categories %}

            <li>{{ item.Name }}</li>

        {% endfor %}

    </ul>

{% else %}

    <p>No Categories Available</p>

{% endif %}

{% endblock %}

Filtering Data

Django allows us to retrieve specific records using filters.

Instead of fetching all records, we can fetch only the records that match a condition.

Example

Category.objects.filter(Name='Technology')

SQL Equivalent

SELECT *
FROM Category
WHERE Name='Technology';

Fetch Single Record

Category.objects.get(Id=1)

SQL Equivalent

SELECT *
FROM Category
WHERE Id = 1;

Ordering Data

Django allows records to be sorted in ascending or descending order.

Ascending Order

Category.objects.all().order_by('Name')

Descending Order

Category.objects.all().order_by('-Name')

SQL Equivalent

SELECT *
FROM Category
ORDER BY Name ASC;

SELECT *
FROM Category
ORDER BY Name DESC;

Quick Revision

Category.objects.all()

Category.objects.filter(Name='Technology')

Category.objects.get(Id=1)

Category.objects.order_by('Name')

Database Table
       ↓
QuerySet
       ↓
View
       ↓
Template
       ↓
Web Page

Assignment