fix: set default value for pinned field on CommentThread to prevent NULL sort bug

[Anas12091101]

Description

This PR sets the default value for the pinned field on CommentThread to prevent the NULL sort bug.

When a new CommentThread is created, the pinned field is set to None (NULL in the database). However, when a thread is pinned and then unpinned, pinned becomes False. This creates an inconsistency: some threads have pinned=None while others have pinned=False, even though both represent "not pinned."

This causes a sorting bug. Thread listing uses the sort order [-pinned, -last_activity_at], and in MySQL, False (0) sorts higher than NULL in descending order. As a result, previously-pinned-then-unpinned threads (pinned=False) appear above never-pinned threads (pinned=None), breaking the expected sort by last_activity_at.

How to test

• Create a new discussion thread via the Discussions MFE or API.
• Verify the thread's pinned value in the database is False, not None. You can check this in the LMS Django admin at /admin/forum/commentthread/.
• Pin the thread, then unpin it. Confirm pinned is still False.
• Create a second thread (do not pin it).
• Set the sort order to "Recent activity" in the Discussions MFE.
• Verify both threads sort correctly by last_activity_at, with neither thread incorrectly jumping above the other due to a pinned value mismatch.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[Anas12091101]

@openedx/wg-maintainers-forums, could someone from the team please take a look at this PR when available?

[taimoor-ahmed-1]

Appreciate the PR @Anas12091101! - Just some thoughts on the migration:

This change makes pinned non-nullable with a default, but the migration does not handle existing NULL values.

If any existing CommentThread.pinned rows are NULL, this may cause issues during migration since setting a default only applies to new rows.

Suggested approach:

1. Add a data migration to backfill existing NULL values to False
2. Then alter the field to be non-nullable with default=False

Example:

def backfill_pinned(apps, schema_editor):
    CommentThread = apps.get_model("forum", "CommentThread")
    CommentThread.objects.filter(pinned__isnull=True).update(pinned=False)

operations = [
    migrations.RunPython(backfill_pinned, migrations.RunPython.noop),
    migrations.AlterField(
        model_name="commentthread",
        name="pinned",
        field=models.BooleanField(default=False),
    ),
]

[Anas12091101]

@taimoor-ahmed-1, thanks for the review. I overlooked this because edX’s default DB provider is MySQL, which automatically converts NULL values to the default during migrations. I’ve now added a backfill function in the migration to ensure compatibility with other databases as well.