#23581: Migration shouldn't generate "DROP DEFAULT" SQL if no database changes
are
required
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Reporter: john_scott | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Migrations | Version: 1.7
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 john_scott):
I would like to contribute a patch for the `DROP DEFAULT` issue if I can.
My previous comments were focused just on the code added @
2fb1939a9efae24560d41fbb7f6a280a1d2e8d06 The issue with that bit of code
appears to be that _alter_field() will almost always output the `DROP
DEFAULT` since:
* all database backends besides MySQL will pass the `not
self.skip_default(new_field)` check
* most fields will also pass the `new_field.default is not None` test
since if no default was actually set `new_field.default` is actually
`django.db.models.fields.NOT_PROVIDED` rather than `None`.
However, now that I've looked into it a bit more I'm uncertain how to
proceed:
* There is code which does more complex evaluation of old/new field's
default situation higher up in _alter_field()
https://github.com/django/django/blob/master/django/db/backends/schema.py#L545.
Should the snippet of code mentioned above be folded in with this section
or is there a reason it was added to near the end of the method?
* The comments in #23043 suggest that no default should ever be set at the
database level but the section L545 definitely does do add defaults at the
db level (see output below).
* Also uncertain why `add_field` would need to drop the default on a newly
created field
https://github.com/django/django/blob/master/django/db/backends/schema.py#L384
Here is the output of `sqlmigrate` after adding a default to a field:
{{{
BEGIN;
ALTER TABLE "accounts_user" ALTER COLUMN "phone" SET DEFAULT
'123-123-1234';
ALTER TABLE "accounts_user" ALTER COLUMN "phone" DROP DEFAULT;
COMMIT;
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/23581#comment:17>
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