How to Prevent Duplicated Scanning of the Same QR Code in Django?

Prevent Duplicated Scanning of the Same QR Code in Django?

To prevent duplicated scanning of the same QR Code in Django, you can use a simple mechanism where you store the list of scanned QR Codes in a database and check against it before allowing a new scan. Here are the steps you can follow.

1. Create a new Django model to store the scanned QR Codes. You can define fields like QR Code content, scan time, and the user who scanned it. For example,

from django.db import models

class ScannedQR(models.Model):
    content = models.CharField(max_length=255, unique=True)
    scan_time = models.DateTimeField(auto_now_add=True)
    scanned_by = models.ForeignKey(User, on_delete=models.CASCADE)

2. Whenever a user scans a QR Code, check if the content already exists in the ScannedQR model. If it does, prevent the new scan and show an error message to the user. If it doesn't, add a new entry to the model with the QR Code content, current time, and the user who scanned it. For example,

from django.utils import timezone
from django.contrib import messages

def scan_qr(request, content):
    # Check if the QR Code has already been scanned
    if ScannedQR.objects.filter(content=content).exists():
        messages.error(request, "This QR Code has already been scanned.")
        return redirect("qr_scan_page")
    else:
        # Add a new entry to the ScannedQR model
        ScannedQR.objects.create(
            content=content,
            scan_time=timezone.now(),
            scanned_by=request.user
        )
        # Redirect to the success page
        messages.success(request, "QR Code scanned successfully.")
        return redirect("qr_success_page")

3. To prevent multiple scans from the same user, you can add a unique constraint to the model fields that identify the user and the QR Code content. For example,

class ScannedQR(models.Model):
    content = models.CharField(max_length=255)
    scan_time = models.DateTimeField(auto_now_add=True)
    scanned_by = models.ForeignKey(User, on_delete=models.CASCADE)
    
    class Meta:
        unique_together = (("content", "scanned_by"),)

With this unique constraint, a user can only scan a particular QR Code once. If they try to scan it again, Django will prevent the duplicate entry and show an error message.

How to Scan Another QR Code Without Relaunching the Camera?

To scan another QR Code without relaunching the camera, you can use a loop that continues to capture images from the camera until a new QR Code is detected. Here is an example of how you can modify your code to achieve this.

import cv2
import requests

# Initialize the camera
cap = cv2.VideoCapture(0)

# Initialize variables to store the QR Code data
last_qr_data = ""

# Loop to continuously capture frames from the camera
while True:
    # Capture a frame from the camera
    ret, frame = cap.read()

    # Decode any QR Codes present in the frame
    qr_data = decode_qr_code(frame)

    # Check if a new QR Code has been detected
    if qr_data != last_qr_data:
        # Post the QR Code data to the server
        post_qr_data(qr_data)

        # Update the last QR Code data variable
        last_qr_data = qr_data

    # Show the frame on screen
    cv2.imshow('frame', frame)

    # Check if the user has pressed the 'q' key to quit the program
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# Release the camera and close the OpenCV window
cap.release()
cv2.destroyAllWindows()

In the above code, we have added a last_qr_data variable to keep track of the last QR Code that was scanned. We then continuously capture frames from the camera using a loop. Within the loop, we check if a new QR Code has been detected by comparing the decoded QR Code data with the last_qr_data variable. If a new QR Code is detected, we post the data to the server using the post_qr_data function and update the last_qr_data variable. Finally, we show the frame on the screen and check if the user has pressed the 'q' key to quit the program.

How to Avoid Duplicate Records in Django?

You can use several approaches to avoid duplicate records in Django. See as follows.

1. Add a unique constraint: You can add a unique constraint to one or more fields in your model. This will ensure that no two records have the same value for those fields. If you try to save a duplicate record, Django will raise a django.db.IntegrityError.

class MyModel(models.Model):
    field1 = models.CharField(max_length=50, unique=True)
    field2 = models.CharField(max_length=50)

    # other fields and methodsss

2. Use get_or_create(): You can use the get_or_create() method to retrieve an existing record or create a new one if it doesn't exist. This method returns a tuple of (object, created), where object is the retrieved or created object, and created is a boolean indicating whether the object was created or retrieved.

obj, created = MyModel.objects.get_or_create(field1='value')

3. Use update_or_create(): You can use the update_or_create() method to update an existing record or create a new one if it doesn't exist. This method returns a tuple of (object, created), where object is the updated or created object, and created is a boolean indicating whether the object was created or updated.

obj, created = MyModel.objects.update_or_create(
    field1='value',
    defaults={'field2': 'new value'}
)

4. Use unique_together: You can use the unique_together attribute to specify a unique constraint across multiple fields.

class MyModel(models.Model):
    field1 = models.CharField(max_length=50)
    field2 = models.CharField(max_length=50)

    class Meta:
        unique_together = ('field1', 'field2')

With these methods, you can ensure that your database records are unique and avoid duplicate records in Django.