python - 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:

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:

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

Answer 1

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.