fix: Preserve author_username and retired_username during MongoDB to …

[blarghmatey]

Summary

This PR fixes the issue where posts from retired users cause the discussions MFE to crash with Missing :learnerUsername param error after migrating from MongoDB to MySQL.

Problem

After migrating forum data from MongoDB to MySQL using the forum_migrate_course_from_mongodb_to_mysql management command, the discussions MFE fails to display posts when clicking on content from retired users.

Error:

Error: Missing ":learnerUsername" param

This occurs in AuthorLabel.jsx when trying to generate a route to the learner's posts page.

Root Cause

1. V1 forum (MongoDB) stores usernames as fields: The original Ruby forum (cs_comments_service) stores author_username and retired_username as separate database fields to preserve historical usernames (see content.rb)

2. V2 migration script ignores these fields: The migration script in migration_helpers.py was not reading author_username or retired_username from MongoDB documents

3. MySQL models derive username at runtime: The V2 MySQL models only stored a foreign key to the User table and derived the username from self.author.username in to_dict() methods

4. Usernames change when users retire: When users are retired, their username is changed to retired__user_<hash>, losing the historical username that was used when content was originally posted

Solution

This PR adds proper username preservation to match the V1 forum behavior:

1. Added Database Fields

• Added author_username field to Content model (stores username at posting time)
• Added retired_username field to Content model (stores name to display for retired users)
• These fields are inherited by CommentThread and Comment models

2. Updated Migration Script

• Modified create_or_update_thread() to read and preserve author_username from MongoDB
• Modified create_or_update_comment() to read and preserve author_username from MongoDB
• Falls back to current username if MongoDB didn't have these fields

3. Updated Models

• Added save() hook to set author_username on content creation for new posts
• Updated to_dict() methods to use stored username with fallback chain:
  • author_username → retired_username → author.username

4. Database Migration

• Created Django migration 0004_add_author_username_fields.py to add the new columns

Testing

• ✅ Posts from active users work as before
• ✅ Posts from retired users now display with correct historical username
• ✅ Frontend properly handles retired user indicators
• ✅ Re-running migration preserves original usernames from MongoDB
• ✅ New posts created in V2 automatically store username

Migration Path for Existing Deployments

For sites that have already migrated courses to MySQL:

1. Apply this PR's code changes
2. Run Django migration: python manage.py migrate forum
3. Re-run content migration for affected courses:

   python manage.py forum_migrate_course_from_mongodb_to_mysql <course_id>

The migration script is idempotent and will update existing records with the preserved usernames.

Related Issues

• Fixes: Forums V2 with retired user results in a display error when viewing posts frontend-app-discussions#817
• Related to PR fix: Retired author username not being fetched #236 (which attempted to fix this but didn't address the root cause)

Additional Context

The V1 forum (Ruby/MongoDB) has always stored usernames as separate fields to avoid issues with username changes. This PR brings that behavior to the V2 forum (Python/MySQL). The investigation revealed that the MongoDB documents already contain the necessary author_username data; we just needed to preserve it during migration.

Checklist

• Added database fields to Content model
• Updated migration script to preserve MongoDB fields
• Updated to_dict() methods with fallback logic
• Created Django migration file
• Documented migration path for existing deployments
• Commit message follows conventional commits format

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