handling field DoesNotExist error within a model class

[Rick Shory]

 I have a model "Channel" with various foreign key fields (e.g. "data_type" below). 

 I have a __unicode__ method to give intelligible information about the model instance by pulling in the foreign key info.

 For example:

    def __unicode__(self):

         ...

        t = self.data_type

         ...

        return u'%s,%s,%s,%s,%s,%s' % (c, l, s, t, u, o)

 (Each of the foreign key models have their own __unicode__ method.)

 This works fine, unless the instance is incomplete:

>>> c = Channel(column = 3)

>>> c

Traceback (most recent call last):

  File "<console>", line 1, in <module>

  File "C:\Python27\lib\site-packages\django\db\models\base.py", line 421, in __

repr__

    u = six.text_type(self)

  File "C:\...\models.py", line 42, in __unicode__

    t = self.data_type

  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line

389, in __get__

    raise self.field.rel.to.DoesNotExist

DoesNotExist 

 How would I correctly write the error handling? This does not quite work:

      

        try:

            t = self.data_type

        except self.data_type.DoesNotExist:

            t = ''

 I've tried various formats for that "except" line, such as:

        except DoesNotExist:

        except self.field.rel.to.DoesNotExist:

        except self.DoesNotExist:

 I can use the general:

        except:

 But I need to learn how to do this.

[Simon Charette]

Use the `DateTypeModelClass.DoesNotExist` exception.

By the way, make sure you use `select_related()` if you want to use `__unicode__` on those instances or it will results in 1 + N * 6 queries where N = `Channel.objects.count()`.