do properties work on django model fields?


Question

I think the best way to ask this question is with some code... can I do this? (edit: ANSWER: no)

class MyModel(models.Model):    
    foo = models.CharField(max_length = 20)    
    bar = models.CharField(max_length = 20)  

    def get_foo(self):  
        if self.bar:  
            return self.bar  
        else:  
            return self.foo  

    def set_foo(self, input):  
        self.foo = input  

    foo = property(get_foo, set_foo)  

or do I have to do it like this:

Yes, you have to do it like this:

class MyModel(models.Model):
    _foo = models.CharField(max_length = 20, db_column='foo')
    bar = models.CharField(max_length = 20)

    def get_foo(self):
        if self.bar:
            return self.bar
        else:
            return self._foo

    def set_foo(self, input):
        self._foo = input

    foo = property(get_foo, set_foo)

note: you can keep the column name as 'foo' in the database by passing a db_column to the model field. This is very helpful when you are working on an existing system and you don't want to have to do db migrations for no reason

1
37
12/7/2018 3:07:23 PM

Accepted Answer

A model field is already property, so I would say you have to do it the second way to avoid a name clash.

When you define foo = property(..) it actually overrides the foo = models.. line, so that field will no longer be accessible.

You will need to use a different name for the property and the field. In fact, if you do it the way you have it in example #1 you will get an infinite loop when you try and access the property as it now tries to return itself.

EDIT: Perhaps you should also consider not using _foo as a field name, but rather foo, and then define another name for your property because properties cannot be used in QuerySet, so you'll need to use the actual field names when you do a filter for example.

20
9/21/2009 3:02:00 PM

As mentioned, a correct alternative to implementing your own django.db.models.Field class, one should use - db_column argument and a custom (or hidden) class attribute. I am just rewriting the code in the edit by @Jiaaro following more strict conventions for OOP in python (e.g. if _foo should be actually hidden):

class MyModel(models.Model):
    __foo = models.CharField(max_length = 20, db_column='foo')
    bar = models.CharField(max_length = 20)

    @property
    def foo(self):
        if self.bar:
            return self.bar
        else:
            return self.__foo

    @foo.setter
    def foo(self, value):
        self.__foo = value

__foo will be resolved into _MyModel__foo (as seen by dir(..)) thus hidden (private). Note that this form also permits using of @property decorator which would be ultimately a nicer way to write readable code.

Again, django will create *_MyModel table with two fields foo and bar.


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