Queryset first() returns different result than queryset[0] when using ExpressionWrapper

[Pablo Pissi]

Hi! I was creating a new app and found the following scenario, seems like an issue to me:

I have three models:

class Transaction(models.Model):
    amount = models.DecimalField(max_digits=10, decimal_places=2)
    description = models.CharField(max_length=255)
    created_by = models.ForeignKey(get_user_model(), on_delete=models.CASCADE, related_name="created_transactions")
    created_at = models.DateTimeField(auto_now_add=True)


class TransactionUser(models.Model):
    user = models.ForeignKey(get_user_model(), on_delete=models.CASCADE, related_name="transactions")
    transaction = models.ForeignKey("bills.Transaction", on_delete=models.CASCADE, related_name="users")
    amount = models.DecimalField(max_digits=10, decimal_places=2)
    transaction_type = models.ForeignKey('bills.TransactionType', on_delete=models.PROTECT)


class TransactionType(models.Model):
    name = models.CharField(max_length=50)
    code = models.CharField(max_length=3)
    multiplier = models.DecimalField(max_digits=5, decimal_places=2)

I create the following instances:

        transaction = Transaction(amount=100)
        positive = TransactionType(multiplier=1)
        negative = TransactionType(multiplier=-1)
        
        TransactionUser(transaction=transaction, user=user1, amount=50, transaction_type=negative)
        TransactionUser(transaction=transaction, user=user2, amount=50, transaction_type=negative)
        TransactionUser(transaction=transaction, user=user1, amount=10, transaction_type=positive)

I want to obtain the balance of each transaction by doing the following query:

 TransactionUser.objects \
        .filter(user_id=user_id) \
        .annotate(
            real_amount=ExpressionWrapper(F("amount")*F("transaction_type__multiplier"), output_field=DecimalField()),
        ).values("transaction") \
        .annotate(
            balance=Sum("real_amount")
        )

If I log the queryset returned, the value logged is:

<QuerySet [{'transaction': 1, 'balance': Decimal('-40.0000')}]>

But if I log

queryset.first()["balance"]
>>> -50

And if I log

queryset[0]["balance"]
>>> -40

It's also reproducible in 4.0

[Simon Charette]

This is happening because you don't explicitly define an ordering for your queryset so ​the database can return whatever rows it wants in the case of [0] while .first() explicitly order by pk ​if no ordering is specified.

Please TicketClosingReasons/UseSupportChannels before filing issues in this ticket tracker.

[Pablo Pissi]

Hi,

I've read the docs about ordering, but in this case the queryset has only one value and the value returned by .first() is not a value that exists on the queryset, is that correct?

It's returning values from the non-aggregated queryset

[Carlton Gibson]

It's also reproducible …

Hi Pablo — it's looks like a usage problem, so the support channels are the place to go, but if you can create a sample project that demonstrates the issue, perhaps in a failing test case, I'm happy to look further.

[Pablo Pissi]

Hi again!

Here is a repo with a test that shows the issue: ​https://github.com/pablop94/django-poc

Let me know if you need more info.

[Simon Charette]

The addition of order_by columns to the GROUP BY clause specified by values().annotate() ​is documented.

The crux of the issue remain; you are asking for an element at a particular index of a collection with an undefined ordering. Adding order_by("example_model") to your test case addresses the irregularity you are experiencing with first().

first() could arguably be adjusted to raise an exception when attempted to be used on an unordered collection to refuse the temptation to guess but that would require a deprecation period. Another approach could be to only raise an exception when first() is called on a unordered queryset with an explicit values() aggregation grouping that doesn't include pk which is the case reported here.

Here's what the code could look like for a systematic error on attempts to use first() on a undefined ordering queryset performing aggregation over specific columns

django/db/models/query.py

@@ -1032 +1032 @@
 
     def first(self):
         """Return the first object of a query or None if no match is found."""
-        for obj in (self if self.ordered else self.order_by("pk"))[:1]:
+        queryset = self
+        if not self.ordered:
+            if isinstance(self.query.group_by, tuple):
+                raise TypeError(
+                    "Cannot use first() on an unordered queryset performing aggregation"
+                )
+            queryset = self.order_by("pk")
+        for obj in queryset[:1]:
             return obj
 
     async def afirst(self):

And a somewhat more complex implementation that still allow the pk

django/db/models/query.py

@@ -1032 +1032 @@
 
     def first(self):
         """Return the first object of a query or None if no match is found."""
-        for obj in (self if self.ordered else self.order_by("pk"))[:1]:
+        queryset = self
+        if not self.ordered:
+            if isinstance(self.query.group_by, tuple) and not any(
+                col.output_field is self.model._meta.pk for col in self.query.group_by
+            ):
+                raise TypeError(
+                    "Cannot use first() on an unordered queryset performing aggregation "
+                    "over a set of expressions that doesn't include the base model's primary key"
+                )
+            queryset = self.order_by("pk")
+        for obj in queryset[:1]:
             return obj
 
     async def afirst(self):

The latter patch seems to be passing the test suite.

Thoughts on the value of such adjustment Carlton?

[Carlton Gibson]

Thanks both. Good.

Thoughts on the value of such adjustment Carlton?

We do see tickets periodically running into this problem: I recall a number of occasions seeing links to that same docs section.
I guess for every one that we see there are several/many in the wild we don't

The first() docs link to the Interaction with order_by() section there, presumably to try to forestall this confusion, but I still get the impression it's one of the more subtle points of the ORM behaviour, so if we can provide a better error message to point folks in the right direction that's likely a win. 🤔

I'll provisionally accept on that basis. (Mariusz can object on review if he doesn't like it 😃)

Pablo would you like to work on a PR based on Simon's suggestion?

[Pablo Pissi]

Yes, I could work on a PR this weekend. Maybe I can add a documentation note to the first function too.

This change will require a deprecation period right?

[Carlton Gibson]

This change will require a deprecation period right?

I don't think so. We're (just) adding a warning for an already incorrect usage.

[Pablo Pissi]

Hi!

I've created this PR. I've also updated the last() function: ​https://github.com/django/django/pull/15769

[Mariusz Felisiak <felisiak.mariusz@…>]

In d2872948:

Fixed #33772 -- Added QuerySet.first()/last() error message on unordered queryset with aggregation.