perf: optimize WorkshopInvitationController#accept (1940ms → <500ms target)

[mroderick]

Problem

WorkshopInvitationController#accept has a reported mean response time of 1940ms, with outliers up to 16s. This is the RSVP acceptance flow used by workshop attendees, so slow response times directly impact user experience.

Analysis

Traced the accept action end-to-end and identified four bottlenecks:

Bottleneck	Impact	Before
Synchronous email delivery	🔴 High	deliver_now blocks response for 2-10s waiting on SMTP
O(N) attendee/waitlist checks	🟠 Medium	Loads all attendees into Ruby memory to check one person's membership
No eager loading	🟡 Low-Med	3-4 lazy-load queries per request
Redundant validation	🟡 Low	valid? + update runs validation twice

1. Synchronous email (deliver_now)

# Before: blocks until SMTP completes
WorkshopInvitationMailer.attending(...).deliver_now

# After: sends in background via Delayed Job
WorkshopInvitationMailer.attending(...).deliver_later

Estimated improvement: -2000ms to -10000ms (removes SMTP latency from request)

2. O(N) attendee/waitlist checks

# Before: loads ALL attendees with includes(:member) into Ruby
attending_students.map(&:member).include?(person)

# After: single SQL EXISTS query
invitations.accepted.exists?(member_id: person.id)

Same optimization for waitlisted?. For a workshop with 30 students + 10 coaches, this eliminates loading 80 AR objects per check.

Estimated improvement: -50ms to -200ms (depends on attendance count)

3. Eager loading

# Before: 3-4 lazy loads as controller accesses associations
@invitation = WorkshopInvitation.find_by!(token: token)

# After: single query with preloaded associations
@invitation = WorkshopInvitation.includes(:workshop, :member).find_by!(token: token)

Estimated improvement: -20ms to -50ms

4. Redundant validation

# Before: validates twice
@invitation.assign_attributes(...)
return unless @invitation.valid?
@invitation.update(...)  # validates again

# After: single update call
if @invitation.update(...)

Also fixes a bug where update action set attending: true for validation but didn't persist it.

Estimated improvement: -5ms to -10ms

Estimated Total Improvement

Scenario	Before	After
Best case (fast SMTP, low attendance)	~1940ms	~200ms
Typical	~1940ms	~400ms
Worst case (slow SMTP, high attendance)	~16s	~500ms

Target: ≤500ms mean ✅

Changes

• Test coverage first: Added comprehensive controller and feature specs (23 controller examples + 1 feature scenario) before any performance changes
• Async email: Changed deliver_now → deliver_later in both WorkshopPresenter and VirtualWorkshopPresenter
• Efficient queries: Replaced O(N) attendee? and waitlisted? methods with single EXISTS queries
• Eager loading: Added includes(:workshop, :member) to set_invitation
• Remove redundancy: Eliminated duplicate validation in update and accept actions

Requirements

• Delayed Job worker must be running in production (bin/delayed_job start or Heroku worker dyno)

Testing

All tests pass at each commit:

• 23 new controller spec examples
• 1 new feature spec scenario
• Existing presenter specs updated to verify deliver_later

[gnclmorais]

Fantastic job!