Peewee: Querying SUM of field/column - peewee

Question

I've been trying to get the sum total of a field/column in peewee. I thought it would be straightforward, but I've been going around in circles for a couple of hours now.
All I'd like to get back from the query is sum total of the price field/column.
An example of the code I've been using is:
Model
class Package(db.Model):
id = PrimaryKeyField()
code = CharField(max_length=11, unique=True, null=False)
price = DecimalField(null=False, decimal_places=2)
description = TextField()
created = DateTimeField(default=datetime.now, null=False)
updated = DateTimeField(default=datetime.now, null=False)
Query
sum_total = fn.SUM(Package.price).alias('sum_total')
query = (Package
.select(sum_total)
.order_by(sum_total)
)
The outputs I'm getting are:
query.sum_total
AttributeError: 'ModelSelect' object has no attribute 'sum_total'
for q in query:
logger.debug(json.dumps(model_to_dict(q)))
{"code": null, "created": null, "description": null, "id": null, "numberOfTickets": null, "price": null, "updated": null}
I've sure I'm missing something really simple. I haven't been able to find any examples outside of the peewee documentation, and I've tried those, but am still getting nowhere.
Any ideas?

Answer 1

The "model_to_dict()" method is not magic. It does not automatically infer that you want to actually just dump the "sum_total" column into a dict. Additionally, are you trying to get a single sum total for all rows in the db? If so this is just a scalar value, so you can write:
total = Package.select(fn.SUM(Package.price)).scalar()
return {'sum_total': total}
If you want to group totals by some other columns, you need to select those columns and specify the appropriate group_by() - for example, this groups sum total by code:
sum_total = fn.SUM(Package.price).alias('sum_total')
query = (Package
.select(Package.code, sum_total)
.group_by(Package.code)
.order_by(sum_total))
accum = []
for obj in query:
accum.append({'code': obj.code, 'sum_total': obj.sum_total})