Django：更改可选ImageField的网址

Question

Via signals I check if my model belongs to a category. If it is, I want to change my optional ImageField to a specific url.

How can this be achieved? Code below doesnt work, I get "cannot set attribute" error as it is an optional field and it was blank as I saved it.

Here is my sample model

class Foo(models.Model):
    category = models.IntegerField(max_length=1)
    poster = models.ImageField(u"Poster", blank=True)

and my post save signal:

def post_poster(instance, **kwargs):
        if instance.category == 1 #a specific category
            instance.poster.url = u'/media/special_image_for_1.png'
            instance.save()
    except MovieCat.DoesNotExist:
        pass 

Answer 1

You don't say what problem you're having (does that code work?), so there are two problems in your code. Firstly, you probably don't want to be saving in a post-save signal (infinite loop, anyone?). Secondly, you've got an indentation issue (you need to indent after the if ).

The way you probably want to do this is with Model.clean() .

Define a clean method on your model like this:

def clean(self):
  if instance.category == 1 #a specific category
    instance.poster.url = u'/media/special_image_for_1.png'