#36632: AlterIndexTogether drops overlapping indexes
-------------------------------------+-------------------------------------
Reporter: mherrmann | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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1. Create a model with two index_together fields: `('a', 'b')`
2. Run `makemigrations`.
3. Add field `c` and change index_together to `(('a', 'b'), ('a', 'b',
'c'))`.
4. Run `makemigrations`.
5. Run `migrate`.
For at least SQLite, this silently drops the two-column index and only
produces the three-column index.
I am attaching code that can be used to reproduce the issue. To run it,
place all files in the same folder, `cd` into it and execute:
{{{
chmod +x steps.sh
./steps.sh
}}}
(This requires macOS or Linux. It should be easy to see how to execute the
steps on Windows.)
Executing `steps.sh` with Django 5 produces error
{{{
ValueError: Found wrong number (0) of indexes for polls_mymodel(a, b).
}}}
On the other hand, executing the script with Django 4 succeeds without any
errors.
(Please see the script for what I mean by "executing with Django 4/5".)
The script generates three migrations. And actually, the bug is already
there after the second migration. But the third migration highlights the
issue with an error message.
I am attaching script `show_indexes.py` that can be used to debug the
issue. It shows the indexes in the SQLite database. When `steps.sh` is
executed with Django 4 and without the third migration, then
`show_indexes.py` displays two indexes - one for two and one for three
columns. On the other hand, when `steps.sh` is executed with Django 5 (and
without the third migration), then `show_indexes.py` only shows one index,
for the three columns. `show_indexes.py` needs Django 4.
--
Ticket URL: <https://code.djangoproject.com/ticket/36632>
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