Django query assigns database columns to wrong model instance properties - django

Question

Really weird problem - when I query for a model instance the data comes back assigned to the wrong properties.
The model:
class SaleLineItem(models.Model):
sale = models.ForeignKey(Sale, on_delete=models.CASCADE, related_name="sale_line_items")
stock_unit = models.ForeignKey(StockUnit, on_delete=models.CASCADE, related_name="sale_line_items")
currency = models.CharField(max_length=3)
price_original = models.FloatField()
price_paid = models.FloatField()
tax_amount = models.FloatField(null=True, blank=True)
num_sold = models.IntegerField()
sale_line_item_id = models.CharField(max_length=30, null=True, blank=True)
status = models.CharField(max_length=20, choices=SALE_STATUS_CHOICES, null=True, blank=True)
The database row:
id | currency | price_original | price_paid | tax_amount | num_sold | sale_line_item_id | status | sale_id | stock_unit_id
-------+----------+----------------+------------+------------+----------+-------------------+-----------+---------+---------------
15726 | THB | 130 | 130 | | 1 | | delivered | 16219 | 2
And the query:
sli = SaleLineItem.objects.get(pk=15726)
print(sli.pk)
-------------------------
16219
print(sli.stock_unit_id)
-------------------------
THB
print(sli.currency)
-------------------------
130.0
The data get populated on the object but everything is "shifted" by one column.
But if I do the query this way:
SaleLineItem.objects.filter(pk=15726).values()
-------------------------
<QuerySet [{'id': 15726, 'sale_id': 16219, 'stock_unit_id': 2, 'currency': 'THB', 'price_original': 130.0, 'price_paid': 130.0, 'tax_amount': None, 'num_sold': 1, 'sale_line_item_id': None, 'status': 'delivered'}]>
. . . the result is correct.
I thought I might have un-migrated models but I ran both makemigrations and migrate to no effect.
Same result when I use lower-level QuerySet methods:
qs = SaleLineItem.objects.all()
clone = qs._chain()
clone.query.add_q(Q(pk=15726))
print(clone)
------------------------------
<QuerySet [<SaleLineItem: SaleLineItem object (16219)>]>
Note the pk on the model __str__ is incorrect.
Any ideas what's happening here?
Running:
Python 3.7.3
Django 2.2.1
Postgres 10

Answer 1

Turns out it's because I overrode __init__ with an extra (non-field) argument.
#classmethod
def from_db(cls, db, field_names, values):
if len(values) != len(cls._meta.concrete_fields):
values_iter = iter(values)
values = [
next(values_iter) if f.attname in field_names else DEFERRED
for f in cls._meta.concrete_fields
]
new = cls(*values)
new._state.adding = False
new._state.db = db
return new
Database values are populated onto the model using *values, and the model expects fields in a specific order. So you can't have an extra argument in __init__ or the order gets messed up.
Edit:
Had not read this part in the docs (https://docs.djangoproject.com/en/2.1/ref/models/instances/):
You may be tempted to customize the model by overriding the __init__
method. If you do so, however, take care not to change the calling
signature . . .