fix: optimize enrollment counts to use read replica and show all configured modes

[brianjbuck-wgu]

Description

Optimizes the enrollment data in the Instructor API v2 Course Info endpoint (CourseInformationSerializerV2) to fix several issues:

1. Show all configured modes: Enrollment counts now include all modes configured for the course via CourseMode.modes_for_course(), even those with 0 enrollments. Modes not configured for the course are omitted. Previously, only modes with at least 1 enrollment appeared in the response.
2. Reduce redundant DB queries: Replaced 3 separate database queries with a single call to the existing CourseEnrollment.objects.enrollment_counts() manager method.
3. Use read replica: The manager method routes queries through use_read_replica_if_available(), reducing load on the primary database.
4. Role-based enrollment counts: Added learner_count (enrollments excluding staff and admins) and staff_count (total minus learner count) to support the enrollment summary UI.

Impacted roles: Course Author, Operator (Instructor Dashboard MFE consumers)

Files changed:

• lms/djangoapps/instructor/views/serializers_v2.py — Refactored get_enrollment_counts() and get_total_enrollment()
• lms/djangoapps/instructor/tests/test_api_v2.py — Updated and added enrollment count tests
• lms/djangoapps/instructor/docs/references/instructor-v2-course-info-spec.yaml — Updated schema to document dynamic mode keys

Supporting information

• Fixes: Instructor Dashboard - Update enrollment data in Instructor API v2 Course Info endpoint #38006
• Slack discussion on enrollment mode display

Other information

• No database migrations required
• No dependencies on other changes
• Backward compatible — the response shape is the same (object with mode keys + total), but now includes additional keys for configured modes that previously had 0 enrollments

[openedx-webhooks]

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[diana-villalvazo-wgu]

in a diff attribute/count, we also need to know how many admins/staff and how many learners are enrolled on the course to have something like this:

[Copilot]

Pull request overview

This PR optimizes enrollment data queries in the Instructor API v2 Course Info endpoint to reduce database load and show all configured course modes. The changes address three issues: showing modes with zero enrollments, reducing redundant queries, and using read replicas. The PR also adds new fields to distinguish between learner and staff enrollment counts.

Changes:

• Refactored enrollment count methods to use the existing CourseEnrollment.objects.enrollment_counts() manager method
• Added learner_count and staff_count fields to separate non-staff enrollments from staff/admin enrollments
• Updated enrollment_counts to include all configured course modes, even those with 0 enrollments

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lms/djangoapps/instructor/views/serializers_v2.py	Refactored enrollment count methods and added learner/staff count fields; removed Count import
lms/djangoapps/instructor/tests/test_api_v2.py	Added tests for new learner/staff counts and for configured vs unconfigured modes; updated existing enrollment count tests
lms/djangoapps/instructor/docs/references/instructor-v2-course-info-spec.yaml	Updated API schema to document new fields and dynamic enrollment_counts structure

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[diana-villalvazo-wgu]

tested on local with this frontend PR openedx/frontend-app-instruct#54 looks good 👍