#33772: Queryset first() returns different result than queryset[0] when using
ExpressionWrapper
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Reporter: pablop94 | Owner: nobody
Type: | Status: new
Uncategorized |
Component: Database | Version: 3.2
layer (models, ORM) | Keywords:
Severity: Normal | ExpressionWrapper,first,queryset
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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
--
Ticket URL: <https://code.djangoproject.com/ticket/33772>
Django <https://code.djangoproject.com/>
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