How to delete special data from database by python django

Question

For example, I have a table called BlackList in the database which looks like this:

and the model of the table is:

class BlackList(models.Model):
    list = models.CharField(max_length=1000, null=True, blank=True)

What I try to do is:

if request.method == "POST":
    username = request.POST.get('username')  # Get username input first
    password = request.POST.get('password')
    user = authenticate(request, username=username, password=password)
    BL = BlackList.objects.values_list('list', flat=True) # Read all data into array
    if username in BL:  # Check if the username is in blacklist
       # Remove this username from the BlackList table

So my question is how to delete the special data for a special, for example, if 'aaa' try to log in, then 'aaa' will be removed or deleted from the BlackList table.

Not sure what the end goal is here, but with above code you will remove the user from the blacklist even if he fails to login. — user1600649
– user1600649, Commented Jul 14, 2020 at 19:11

I_Can_Help · Accepted Answer · 2020-07-14 17:23:46Z

2

There is .delete method. I. e. try this:

if BlackList.filter(list=username).exists(): 
    BlackList.objects.get(list=username).delete()

instead of:

BL = BlackList.objects.values_list('list', flat=True) # Read all data into array 
if username in BL: # Check if the username is in blacklist 
    # Remove this username from the BlackList table

You can read more about that on https://docs.djangoproject.com/en/3.0/topics/db/queries/#deleting-objects and https://docs.djangoproject.com/en/3.0/ref/models/querysets/#exists

edited Jul 14, 2020 at 17:23

answered Jul 14, 2020 at 17:13

I_Can_Help

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Preeti Y. · Accepted Answer · 2020-07-14 17:40:20Z

2

You can use filter() in combination with first() to get the exist username:

blacklist = Blacklist.objects.filter(username=username).first()

(This will return None if there is no match. If you use get(), you will get DoesNotExist error instead -- which is not preferred.)

After that, you can just delete it:

blacklist.delete()

It will take 2 queries (get and delete) to achieve your goal.

An alternative way is to delete it without getting the object:

Blacklist.objects.filter(username=username).delete()

This statement will be execute with only one query which is equals DELETE FROM ... WHERE username='username'

answered Jul 14, 2020 at 17:40

Preeti Y.

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