Calling methods from models.py inside views.py without creating instance

Question

New to Django coming from .NET with an architectural question.

Inside my models.py, I have a concept called city. These cities can be enabled/disabled.

Inside my views, I want to retrieve all active cities under my view called Cities. I need to retrieve all active cities in many places, so I thought I'd make a method inside my models.py city class called get_in_country, so it looks like this:

class City(models.Model):
    title = models.CharField(max_length=200)
    alias = models.CharField(max_length=200)
    country = models.ForeignKey(Country, null=True)
    is_visible = models.BooleanField(default=False)

    def __str__(self):
        return self.title

    def get_in_country(self, country_id):
        #return best code ever seen

Anyway, my question now is: how do I use this inside views.py?

Being an awesome noob, I of course tried this:

def country(request, alias):
    cities_in_country = City.get_in_country(1) #whatever id

    data = {
            'cities_in_country': cities_in_country, 
        }

    return render(request, 'country.html', data)

Now, you don't have to be Einstein (ahem, Jon Skeet?) to realize this will go wrong, as I haven't made an instance of City and will cause an exception:

unbound method get_in_country() must be called with City instance as first argument (got int instance instead)

So: how would you modify my code to use my new awesome submethod?

Since you want to filter all rows in the City table by Country, a custom Manager method like City.objects.filter_by_country(country) would be best pratice. — cbergmiller
– cbergmiller, Commented May 15, 2015 at 6:38

nu11p01n73R · Accepted Answer · 2015-05-15 05:45:54Z

1

You need to define get_in_country as a static function

By adding a decorator

@staticmethod

just before the class defenition as

@staticmethod 
    def get_in_country(self, country_id):

class City(models.Model):
    title = models.CharField(max_length=200)
    alias = models.CharField(max_length=200)
    country = models.ForeignKey(Country, null=True)
    is_visible = models.BooleanField(default=False)

    def __str__(self):
        return self.title

    @staticmethod # Changed here
    def get_in_country(self, country_id):

edited May 15, 2015 at 5:45

answered May 15, 2015 at 5:43

nu11p01n73R

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5 Comments

Lars Holdgaard Over a year ago

Ah, static methods. The methods I always avoid in .NET ;-) When doing a static method, I assume you remove "self" ? If not, what do you provide as self?

nu11p01n73R Over a year ago

You need to provide any self as argument. If you need to acess some class variables, pass the instance instead

Lars Holdgaard Over a year ago

But lets say my new static method is 'def get_in_country(self, country_id):', and I call it by saying City.get_in_country(current_country.id) - I will get an "get_in_country() takes exactly 2 arguments (1 given)" error. How to fix that?

nu11p01n73R Over a year ago

@LarsHoldgaard No, you should be loosing self altogether. That is the def must look like def get_in_country(country_id):. For example ideone.com/wLA3g8

Lars Holdgaard Over a year ago

Great! Thanks a lot - it makes sense! :) Really a big help!

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