>>> from django.db.models import models
>>> class MarketPrice(models.Model):
market = models.CharField(max_length=30)
crop = models.CharField(max_length=30)
price = models.PositiveSmallIntegerField()
date = models.DateField()
def __str__(self):
return "{}, {}, {}, {}".format(
self.market,
self.crop,
self,price,
self.date
)
>>> FIXTURES = [
('London', 'carrots', 15, datetime.date(2015, 1, 1)),
('London', 'carrots', 20, datetime.date(2015, 1, 2)),
('London', 'potatoes', 12, datetime.date(2015, 1, 1)),
('London', 'potatoes', 14, datetime.date(2015, 1, 2)),
('Manchester', 'carrots', 18, datetime.date(2015, 1, 1)),
('Manchester', 'carrots', 21, datetime.date(2015, 1, 2)),
('Manchester', 'potatoes', 10, datetime.date(2015, 1, 1)),
('Manchester', 'potatoes', 12, datetime.date(2015, 1, 2)),
]
>>> for market, crop, price, date in FIXTURES:
MarketPrice.objects.create(market=market,
crop=crop,
price=price,
date=date)
We want to get only the latest prices for every possible combination of
markets and crops...
>>> prices = (MarketPrice.objects
.order_by('market', 'commodity', '-date')
.distinct('market', 'commodity'))
[<MarketPrice: London, carrots, 20, 2015-01-02>, <MarketPrice: London,
potatoes, 14, 2015-01-02>, <MarketPrice: Manchester, carrots, 21, 2015-01-02
>, <MarketPrice: Manchester, potatoes, 12, 2015-01-02>]
.filter() works as expected
>>> prices.filter(market='Manchester', crop='carrots')
[<MarketPrice: Manchester, carrots, 21, 2015-01-02>]
but .get is totally unexpected
>>> prices.get(market='Manchester', crop='carrots')
<MarketPrice: Manchester, carrots, 18, 2015-01-01>
not only does it return the 'wrong' entry, but one which didn't even seem
to be in prices (because it's a distinct queryset)
It looks and feels even weirder if you apply the filtering first:
>>> prices = (MarketPrice.objects
.filter(market='Manchester', crop='carrots')
.order_by('market', 'commodity', '-date')
.distinct('market', 'commodity'))
>>> prices
[<MarketPrice: Manchester, carrots, 21, 2015-01-02>]
>>> prices.get(market='Manchester', crop='carrots')
<MarketPrice: Manchester, carrots, 18, 2015-01-01>
There's a note seemingly related to this at
https://docs.djangoproject.com/en/1.7/ref/models/querysets/#django.db.models.query.QuerySet.distinct,
but I'm not clear how it directly describes what I'm seeing here.
The line which seems to be causing this is from
https://github.com/django/django/commit/e4757ec7afd54861e0c34d9b0f5edbbac4e2b860,
which strips the ordering I've previously applied
>>> prices
[<MarketPrice: Manchester, carrots, 21, 2015-01-02>]
>>> prices.order_by()
[<MarketPrice: Manchester, carrots, 18, 2015-01-01>]
Is this intended behaviour?
PS the above code examples are untested, manually-typed pseudocode for the
sake of keeping the example simple.
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