Django custom model Manager database error - django

Question

I'm trying to build a custom model manager, but have run into an error. The code looks like this:
class LookupManager(models.Manager):
def get_options(self, *args, **kwargs):
return [(t.key, t.value) \
for t in Lookup.objects.filter(group=args[0].upper())]
class Lookup(models.Model):
group = models.CharField(max_length=1)
key = models.CharField(max_length=1)
value = models.CharField(max_length=128)
objects = LookupManager()
(I have played around with get_options quite a lot using super() and other ways to filter the results)
When I run syncdb, I get the following error (ops_lookup being the corresponding table):
django.db.utils.DatabaseError: no such table: ops_lookup
I noticed that if I change the manager to return [] instead of a filter, then syncdb works. Also, if I've run syncdb and all the tables exist, then change the code to the above, it works as well.
How can I get Django to not expect this table to exist when running syncdb for the first time?
Update
After looking through the traceback I realised what was happening. The lookup table is meant to contain values which populate the choices of some columns in other tables. I think what happens is that the manager gets called when the other tables are created which, it seems, happens before the lookup table is created.
Is there any way to force django to create the lookup table first (short of renaming it?)

Answer 1

What's happening is that you're trying to access the database during module load time. For example:
class MyModel(models.Model):
name = models.CharField(max_length=255)
class OtherModel(models.Model):
some_field = models.CharField(
max_length=255,
# Next line fails on syncdb because the database table hasn't been created yet
# but the model is being queried during module load time (during class definition)
choices=[(o.pk, o.name) for o in MyModel.objects.all()]
)
This is equivalent to what you're doing because, as you've stated, you're using the manager method (transitively) to generate choices for other models.
Replacing the list comprehension with a generator expression will return an iterable, but will not evaluate the filtered queryset until the first iteration. So, this would fix the above example:
choices=((o.pk, o.name) for o in MyModel.objects.all())
Using your example, it would be:
class LookupManager(models.Manager):
def get_options(self, *args, **kwargs):
return ((t.key, t.value) for t in Lookup.objects.filter(group=args[0].upper()))
(note the use of ( and ) instead of [ and ]) (the outer ones) - that is the syntax for creating a generator expression.