Welcome back! After learning how to create relationships between models, it's time to ensure the integrity and quality of the data being stored. In this lesson, we will focus on validating data within your Django applications. Validation is a crucial step to ensure that the data flowing into your database meets certain standards and rules.

By the end of this lesson, you will be proficient in:

• Adding custom validation rules to your Django models
• Handling validation errors within your views

To get started, let's create a validation function for our Todo model to ensure that the task description is at least three characters long. Here's how you can add custom validation in your models.py file:

Python
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1from django.db import models
2from django.core.exceptions import ValidationError
3
4def validate_task(value):
5    if len(value) < 3:
6        raise ValidationError('Task must be at least 3 characters long.')
7
8class Todo(models.Model):
9    task = models.CharField(max_length=200, validators=[validate_task])
10
11    def __str__(self):
12        return self.task

In this example, the validate_task function checks if the length of the task is less than three characters and raises a ValidationError if the condition is met.

Next, let's incorporate this validation into the add_todo view:

Python
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1from django.http import JsonResponse
2from django.views.decorators.csrf import csrf_exempt
3from .models import Todo
4from django.core.exceptions import ValidationError
5import json
6
7def add_todo(request):
8    if request.method == 'POST':
9        data = json.loads(request.body)
10        new_todo = Todo(task=data['task'])
11        try:
12            new_todo.full_clean()
13            new_todo.save()
14            return JsonResponse({'id': new_todo.id, 'task': new_todo.task}, status=201)
15        except ValidationError as e:
16            return JsonResponse({'message': str(e)}, status=400)
17    return JsonResponse({'message': 'Invalid request'}, status=400)

In this view, the full_clean method ensures that the model instance is validated before it is saved to the database. If a validation error occurs, it is caught and returned in the response.

After adding these changes, we can test the validation by sending a POST request with a task that is less than three characters long, for example {"task": "Do"}. You should receive a 400 Bad Request response with the message Task must be at least 3 characters long..

Data validation is an essential part of building reliable and user-friendly web applications. By enforcing data integrity rules, you can:

• Prevent invalid data from entering your database
• Enhance the user experience by providing immediate feedback on data entry errors
• Ensure that your application behaves as expected

Exciting, right? Now it's time to put these concepts into practice. Let's move on to the practice section and start validating data together!