Django - Checking if objects exists and raising error if it does

Question

I need to check if an object exists in a table. If the object doesn't exist I want to save it, if it does exist I want to refresh the page and tell the user that the identifier (quote number) already exists. I have this example code:

def some_method(request):
    if request.method == 'POST':
        form = SomeForm(request.POST)
        if form.is_valid:
            quote = request.POST.get('quote')
            if SomeModel.objects.get(quote_number = quote).exists():
               refresh with error #not sure how to do
            else:
               save object #I can do this part

The problem I am running into is that when I check if the object exists (and it does) then the code will raise an error saying the object doesn't exist before it hits the if. Which means on the webpage is full of coding information rather than the refresh with a message for the user.

I want to be able to have a little pop up message or something for the user to enter a new quote number rather than having the developer error page show up.

Am I going about this the correct way?

Does SomeForm involve the quote at all? ideally that shouldn't be valid if it really isn't — Sayse
– Sayse, Commented Feb 13, 2017 at 14:26
@Sayse SomeForm is a form for posting information about a quote. It's considered "valid" if all the fields are entered correctly. I need to check before the information is save, if the quote number (which is the unique identifier) has already been put into the table. — GreenSaber
– GreenSaber, Commented Feb 13, 2017 at 14:29
You could use the messages framework for the "showing the error" part. docs.djangoproject.com/en/1.10/ref/contrib/messages — Chris
– Chris, Commented Feb 13, 2017 at 14:38

Moses Koledoye · Accepted Answer · 2017-02-13 14:35:20Z

4

The problem is get returns an object, but exists only works with querysets. A Model.DoesNotExist error will be raised when you use get and the object does not exist.

You should use filter instead of get:

qs = SomeModel.objects.filter(quote_number = quote)
if qs.exists():
    ...
else:
    ...

edited Feb 13, 2017 at 14:35

answered Feb 13, 2017 at 14:31

Moses Koledoye

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1 Comment

GreenSaber Over a year ago

I didn't realize that .exists() only works on querysets. I just checked the code and it works perfectly. Thank you for your quick answer!

4140tm · Accepted Answer · 2017-02-13 14:41:10Z

2

I would do something like this:

from django.http import HttpResponseRedirect
from django.contrib import messages

...

try:
    some_object = SomeModel.objects.get(quote_number=quote)
    message.warning(request, 'some message')
    return HttpResponseRedirect('some-url')
except SomeModel.DoesNotExist:
    some_object = SomeModel.objects.create(quote_number=quote)
    ...

And in the template you can display the message just like so:

{% if messages %}
    {% for message in messages %}
        {{message}}
    {% endfor %}
{% endif %}

answered Feb 13, 2017 at 14:41

4140tm

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Comments

dmvrtx · Accepted Answer · 2017-02-13 14:46:20Z

0

You can use get_or_create. It returns a tuple with object retrieved (or created) and a boolean value. If boolean value is True when it's a new object, and if it's False when object already exists and you can throw desired error.

answered Feb 13, 2017 at 14:46

dmvrtx

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