Remove mongodb backend

[salman2013]

Summary

Remove the MongoDB backend from the openedx/forum service entirely. MySQL is now the only supported backend. All data migration tooling (MongoDB
→ MySQL commands, helpers, and tests) is preserved so operators can still migrate existing MongoDB data before fully decommissioning their
MongoDB deployment.

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Motivation

openedx/public-engineering#424

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What Was Removed

MongoDB backend package (forum/backends/mongodb/) — deleted entirely:

• api.py — MongoBackend class
• base_model.py — MongoBaseModel with MongoDB connection handling
• comments.py, threads.py, contents.py, subscriptions.py, users.py — MongoDB collection models

Backend routing logic (forum/backend.py) — simplified from conditional flag-based routing to always returning MySQLBackend:

Waffle flag (forum/toggles.py) — deleted. ENABLE_MYSQL_BACKEND CourseWaffleFlag and enable_mysql_backend_for_course() are no longer needed since
MySQL is always used.

MongoDB index creation command (forum/management/commands/forum_create_mongodb_indexes.py) — deleted. Only relevant when running MongoDB.

MongoDB-specific Elasticsearch import path (forum/search/es.py):

• Removed _import_to_es_from_mongo() method
• Removed get_index_model_rel() method (returned MongoDB model instance)
• Removed MongoDB model imports from forum.backends.mongodb
• models property and rebuild_indices()/catchup_indices() now use MySQL models only

MongoDB-specific API routing (forum/api/threads.py, forum/api/comments.py):

• get_course_id_by_thread() and get_course_id_by_comment() no longer fall back to MongoDB search; MySQL-only

MongoDB backend tests (tests/test_backends/test_mongodb/) — deleted entirely

MongoDB-specific thread view tests (tests/test_views/test_threads.py):

E2E docker-compose (tests/e2e/docker-compose.yml) — removed mongodb service block

Test fixtures (tests/conftest.py):

• Removed patch_default_mongo_database autouse fixture
• Removed parametrized patched_get_backend (previously ran all tests against both backends)
• Removed patched_mongo_backend autouse fixture
• Simplified patched_get_backend to return MySQLBackend directly

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What Was Kept (Intentionally)

Migration commands (operators still need these to migrate existing MongoDB data):

• forum/management/commands/forum_migrate_course_from_mongodb_to_mysql.py
• forum/management/commands/forum_delete_course_from_mongodb.py
• forum/migration_helpers.py — all migration logic intact

MongoDB connectivity (needed by migration commands):

• forum/mongo.py — get_database() function
• pymongo in requirements/base.in
• mongomock in requirements/test.in
• FORUM_MONGODB_DATABASE and FORUM_MONGODB_CLIENT_PARAMETERS settings

MongoContent MySQL model (forum/backends/mysql/models.py) — tracks MongoDB ObjectId → MySQL PK mappings used during migration; the corresponding
MongoContentAdmin in forum/admin.py is also kept

Migration tests (tests/test_management/test_commands/test_migration_commands.py) — all kept; only removed the patch_enable_mysql_backend fixture
that mocked the now-deleted enable_mysql_backend_for_course()

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Key Code Changes

forum/search/es.py

• models property returns mysql_model_indices (was mongo_model_indices)
• mysql_models property kept unchanged
• rebuild_indices() uses get_mysql_model_from_index_name() (previously called get_index_model_rel())
• No method renames; only MongoDB-specific methods removed

forum/migration_helpers.py

• Removed only enable_mysql_backend_for_course() function (sole dependency on deleted toggles.py)
• All migrate_users, migrate_content, migrate_read_states, and related helpers untouched

forum/management/commands/forum_migrate_course_from_mongodb_to_mysql.py

• Removed --no-toggle / -T argument
• Removed enable_mysql_backend_for_course(course_id) call
• Core migration loop unchanged

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Testing Impact

• All existing MySQL backend tests pass unchanged
• Migration command tests pass unchanged
• E2E tests pass unchanged
• MongoDB backend tests deleted (no longer applicable)

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Ticket:
openedx/public-engineering#502

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[taimoor-ahmed-1]

Changes look good.

However, I would also suggest to drop the ENABLE_MYSQL_BACKEND waffle flag too in this PR.

[salman2013]

@taimoor-ahmed-1 PR is available for review. Thanks

[taimoor-ahmed-1]

Code changes look good. Can you also update README to remove all references to mongo backend and toggle.
Also, please bump up the version in the same PR.

[taimoor-ahmed-1]

LGTM 🚀