[Django] #23678: Database default not removed if default=None

[Django]

#23678: Database default not removed if default=None
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Reporter: timgraham | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7
Severity: Release blocker | 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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If you have a model field like `models.CharField(max_length=255,
blank=True, null=True, default=None)`, you will end up something like
`varchar(255) DEFAULT NULL` with MySQL. The `DEFAULT NULL` should be
dropped but isn't due to the `field.default is not None` check here:
https://github.com/django/django/blob/8d6e1afe0be0570253a508f43e2ef89ae640984e/django/db/backends/schema.py#L382

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Ticket URL: <https://code.djangoproject.com/ticket/23678>
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[Django]

#23678: Database default not removed if default=None

---------------------------------+------------------------------------

Reporter: timgraham | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7

Severity: Release blocker | Resolution:

Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0

---------------------------------+------------------------------------

Comment (by paulcdejean):

Not exactly accurate:

* Every varchar column will have DEFAULT NULL if it's created on the
inital migration, regardless of the value of the "default" keyword
argument.
* Every varchar column created as part of a non initial migration will
have DEFAULT NULL if the "default" keyword argument is equal to None.
* A varchar column that has its attributes changed that is DEFAULT NULL in
the database will revert from DEFAULT NULL to no default if the "default"
keyword argument is not None.
* A varchar column that has its attributes changed that is DEFAULT NULL in
the database will remain DEFAULT NULL if the "default" keyword argument is
None.
* A varchar column that has no default in the database will continue to
have no default regardless of its "default" keyword argument. Meaning it
will not at any point go back to DEFAULT NULL.

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Ticket URL: <https://code.djangoproject.com/ticket/23678#comment:1>

[Django]

#23678: Database default not removed if default=None

---------------------------------+------------------------------------

Reporter: timgraham | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7

Severity: Release blocker | Resolution:

Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0

---------------------------------+------------------------------------

Comment (by loic):

@paulcdejean how do you get those results? I just tried with the
`sqlmigrate` command on initial migration on MySQL and my
`models.CharField(max_length=42, null=True)` doesn't show a DEFAULT.

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Ticket URL: <https://code.djangoproject.com/ticket/23678#comment:2>

[Django]

#23678: Database default not removed if default=None

---------------------------------+--------------------------------------
Reporter: timgraham | Owner: nobody
Type: Bug | Status: closed
Component: Migrations | Version: 1.7
Severity: Release blocker | Resolution: worksforme

Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0

---------------------------------+--------------------------------------
Changes (by carljm):

* status: new => closed
* resolution: => worksforme

Comment:

Replying to [comment:2 loic]:

> @paulcdejean how do you get those results? I just tried with the
`sqlmigrate` command on initial migration on MySQL and my
`models.CharField(max_length=42, null=True)` doesn't show a DEFAULT.

You can see the difference as described if you actually run the migration
and then check `show create table tablename` in MySQL.

However, I am not convinced that this reflects a bug in Django, as opposed
to just a slightly odd behavior of `show create table` in MySQL. (I am not
very familiar with MySQL, so I could well be missing something).

For one thing, this difference doesn't show up in `describe tablename`:
all nullable fields with no other default show `NULL` in the "default"
column, regardless of whether they show `DEFAULT NULL` in the `show create
table tablename` output or not.

Secondly, if I add a column directly in MySQL (no involvement from Django)
using this exact syntax: `ALTER TABLE 'testapp_thing' ADD COLUMN 'direct'
varchar(100);` (note that I did not specify `DEFAULT NULL`, or any other
default), this is what `show create table testapp_thing` gives for that
column: `'direct' varchar(100) DEFAULT NULL,`. In other words, MySQL seems
to be adding that `DEFAULT NULL` itself, even if it wasn't in the `ALTER
TABLE`.

And lastly, I am not convinced that any of this matters anyway. The
nullable columns that do and don't show `DEFAULT NULL` in the `show create
table tablename` output seem to behave identically - they all in practice
still use `NULL` as their default value (consistent with the fact that
they all show `NULL` in the "default" column of their `describe` output).

My conclusion: `NULL` is the "default default" for any nullable column
without another explicit default value. There is no issue in Django here,
just some strange (but harmless) behavior from MySQL.

Closing; please reopen if you can provide further evidence that this is a
problem in Django.

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Ticket URL: <https://code.djangoproject.com/ticket/23678#comment:3>