#36051: Count CompositePrimaryKey field targets toward function arity check
-------------------------------------+-------------------------------------
Reporter: Jacob Walls | Owner: Jacob
| Walls
Type: New feature | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by Simon Charette):
* cc: Simon Charette (added)
Comment:
I understand the desire to streamline the composite primary key reference
experience but this approach just seems plain wrong. Function arity is
tied to the number of argument passed to a function and `Max(F("pk"))` is
a single argument where the argument itself is composite.
In other words, there is a difference between calls of the form `foo(1,
2)` and `foo((1,2))` (or `foo(bar)` where `bar: int | tuple[int, int]`
since `F` is a reference) and trying to automatically turn one into the
other seems like it's going to cause more harm than good.
What about we merge both #36042 and this ticket under a single one that
adds a `BaseExpression.allows_composite_expression: bool = False` that we
set to `True` on `Count` and `TupleLookupMixin` and we make
`BaseExpression.resolve_expression` raise a `ValueError` when it's set to
`False` and any of its source expression is an instance of `ColPairs`?
Don't get me wrong I think we should define `arity` for `Aggregate`
subclasses to guard against improper calls but I don't think that making
`Func.resolve_expression` unpack composite expressions is something we
should do for the aforementioned reasons.
--
Ticket URL: <https://code.djangoproject.com/ticket/36051#comment:3>
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