Django check if value exists in the database and create and save if not

Question

I need to check if a value already exists in the database, if it already exists I can use that value, else I have to create the values, save them to the database and show them to the screen.

def currency_rates(request):
    currency_pairs_values = []
    currency_pairs = CurrencyPair.objects.filter(default_show__exact=True)
    for currency_pair in currency_pairs:
        if not CurrencyPairHistory.objects.get(currency_pair__exact=currency_pair,
                                               date__exact=datetime.now().date()).exists():
            currency_pair_history_value = CurrencyPairHistory()
            currency_pair_history_value.currency_pair = currency_pair
            currency_pair_history_value.currency_pair_rate = currency_pair.calculate_currency_pair(
                datetime.now().date())
            currency_pair_history_value.date = datetime.now().date()
            currency_pair_history_value.save()
            currency_pairs_values.append(currency_pair_history_value)
        else:
            currency_pairs_values.append(CurrencyPairHistory.objects.get(currency_pair__exact=currency_pair,
                                                                         date__exact=datetime.now().date()).exists())

    context = {
        'currency_pairs_values': currency_pairs_values
    }

    return render(request, '../templates/client/currencypairs.html', context)

I got the idea for using the exists() method from this link: How to check if something exists in a postgresql database using django? When using this code I get an error DoesNotExist at /currencypairs/

This is the full stacktrace

Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/currencypairs/

Django Version: 1.8.6
Python Version: 3.4.3
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'client']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware']


Traceback:
File "/home/johan/sdp/currency-converter/lib/python3.4/site-packages/django/core/handlers/base.py" in get_response
  132.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/johan/sdp/currency-converter/currency_converter/client/views.py" in currency_rates
  36.                                                date__exact=datetime.now().date()).exists():
File "/home/johan/sdp/currency-converter/lib/python3.4/site-packages/django/db/models/manager.py" in manager_method
  127.                 return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/home/johan/sdp/currency-converter/lib/python3.4/site-packages/django/db/models/query.py" in get
  334.                 self.model._meta.object_name

Exception Type: DoesNotExist at /currencypairs/
Exception Value: CurrencyPairHistory matching query does not exist.

I hope someone will be able to help me out here. Thanks in advance.

Solution 1

You can use the get_or_create() method:

obj, created = MyModel.objects.get_or_create(first_name='John', last_name='Lennon')

This could return:

1. If it already exist:
   • obj: The object from your DB
   • created: False
2. If it does not exist:
   • obj: The new created object
   • created: True

Solution 2

Django has a get or create.

An example would be:

obj, created = CurrencyPairHistory.objects.get_or_create(currency_pair=currency_pair, date=datetime.now().date())
currency_pairs_values.append(obj)