Pseudo-random ordering in django queryset - django

Question

Suppose I have a QuerySet that returns 10 objects, 3 of which will be displayed in the following positions:
[ display 1 position ] [ display 2 position ] [ display 3 position ]
The model representing it is as follows:
class FeaturedContent(models.Model):
image = models.URLField()
position = models.PositiveSmallIntegerField(blank=True, null=True)
where position can be either 1, 2, 3, or unspecified (Null).
I want to be able to order the QuerySet randomly EXCEPT FOR the objects with a specified position. However, I can't order it by doing:
featured_content = FeaturedContent.objects.order_by('-position', '?')
Because if I had one item that had position = 2, and all the other items were Null, then the item would appear in position 1 instead of position 2.
How would I do this ordering?
Thinking about this, perhaps it would be best to have the data as a dict instead of a list, something like:
`{'1': item or null, '2': item or null, '3': item or null, '?': [list of other items]}`

Answer 1

If you use a db backend that does random ordering efficiently you could do it like this:
# This will hold the result
featured_dict = {}
featured_pos = FeaturedContent.objects.filter(position__isnull=False).order_by('-position')
featured_rand = FeaturedContent.objects.filter(position__isnull=True).order_by('?')
pos_index = 0
rand_index = 0
for pos in range(1, 4):
content = None
if pos_index < len(featured_pos) and featured_pos[pos_index].position == pos:
content = featured_pos[pos_index]
pos_index += 1
elif rand_index < len(featured_rand):
content = featured_rand[rand_index]
rand_index += 1
featured_dict[str(pos)] = content
# I'm not sure if you have to check for a valid index first before slicing
featured_dict['?'] = featured_rand[rand_index:]

Answer 2

If you just want to iterate over the queryset you can have two querysets, order them and chain them.
import itertools
qs1 = FeaturedContent.objects.filter(position__isnull=False).order_by('-position')
qs2 = FeaturedContent.objects.filter(position__isnull=True).order_by('?')
featured_content = itertools.chain(qs1, qs2)
for item in featured_content:
#do something with qs item
print item
Upadate:
Since you are asking to make sure position determines the order and the "blank" spaces are replaced randomly by elements with null positions. If the featured list you want to obtain is not too large, 20 in this case
featured = []
rands = []
for i in xrange(1, 20):
try:
x = FeaturedContent.objects.get(position=i) # assuming position is unique
except FeaturedContentDoesNotExist:
if not rands:
rands = list(FeaturedContent.objects.filter(position__isnull=True).order_by('?')[:20]
x = rands[0]
rands = rands[1:]
featured.append(x)

Answer 3

I would post process it, doing a merge sort between the ordered and unordered records.
EDIT:
The beginnings of a generator for this:
def posgen(posseq, arbseq, posattr='position', startpos=1):
posel = next(posseq)
for cur in itertools.count(startpos):
if getattr(posel, posattr) == cur:
yield posel
posel = next(posseq)
else:
yield next(arbseq)
Note that there are lots of error conditions possible in this code (hint: StopIteration).