get_or_create generic relations in Django & python debugging in general
- by rabidpebble
I ran the code to create the generically related objects from this demo:
http://www.djangoproject.com/documentation/models/generic_relations/
Everything is good intially:
>>> bacon.tags.create(tag="fatty")
<TaggedItem: fatty>
>>> tag, newtag = bacon.tags.get_or_create(tag="fatty")
>>> tag
<TaggedItem: fatty>
>>> newtag
False
But then the use case that I'm interested in for my app:
>>> tag, newtag = bacon.tags.get_or_create(tag="wholesome")
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/usr/local/lib/python2.6/dist-packages/django/db/models/manager.py", line 123, in get_or_create
return self.get_query_set().get_or_create(**kwargs)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/query.py", line 343, in get_or_create
raise e
IntegrityError: app_taggeditem.content_type_id may not be NULL
I tried a bunch of random things after looking at other code:
>>> tag, newtag = bacon.tags.get_or_create(tag="wholesome", content_type=TaggedItem)
ValueError: Cannot assign "<class 'generics.app.models.TaggedItem'>": "TaggedItem.content_type" must be a "ContentType" instance.
or:
>>> tag, newtag = bacon.tags.get_or_create(tag="wholesome", content_type=TaggedItem.content_type)
InterfaceError: Error binding parameter 3 - probably unsupported type.
etc.
I'm sure somebody can give me the correct syntax, but the real problem here is that I have no idea what is going on. I have developed in strongly typed languages for over ten years (x86 assembly, C++ and C#) but am new to Python. I find it really difficult to follow what is going on in Python when things like this break.
In the languages I mentioned previously it's fairly straightforward to figure things like this out -- check the method signature and check your parameters. Looking at the Django documentation for half an hour left me just as lost. Looking at the source for get_or_create(self, **kwargs) didn't help either since there is no method signature and the code appears very generic. A next step would be to debug the method and try to figure out what is happening, but this seems a bit extreme...
I seem to be missing some fundamental operating principle here... what is it? How do I resolve issues like this on my own in the future?
