#33319: Query.change_aliases raises an AssertionError
-------------------------------------+-------------------------------------
Reporter: Ömer Faruk Abacı | Owner: Vishal
| Pandey
Type: Bug | Status: assigned
Component: Database layer | Version: 3.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
Ömer,
> My initial attempt was to use Query.bump_prefix but as you told it threw
an error.
The error you encountered is due to `bump_prefix` requiring some
adjustments. The alias merging algorithm that `Query.combine` uses
requires that both query share the same initial alias in order to work
otherwise you'll get a `KeyError`. You'll want to find a way to have
`bump_prefix` adjust all the aliases but the initial one so it can still
be used as the merge starting point.
---
Vishal,
> Here, I am randomly choosing an uppercase letter as the first character
of alias, this returns different aliases, but a test_case is failing with
this approach, with the following traceback.
Choosing a random initial alias will bring more bad than good as not
relying on `alias_prefix` will cause undeterministic failures across the
board. There are reasons why the prefix is used and bumped only on
conflic; it makes reasoning about possible collisions easier.
> In 2nd approach, I have rotated the alias_prefix in a circular manner
from T(class variable alias_prefix has value T) to Z, then from A to Z,
and so on, and it also works on the above-attested code but fails five
other test cases.
This approach doesn't account for possible collisions with subquery
aliases and will eventually collide once you've wrapped up around the 26
letter of the alphabet instead of using the Cartesian product approach.
---
All of the required logic for preventing `alias_prefix` collisions already
lives in `bump_prefix`, it's only a matter of adjusting it in order to
allow it to be used in the alias merging algorithm of `combine`.
--
Ticket URL: <https://code.djangoproject.com/ticket/33319#comment:8>
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