Avoid preloading patient sessions

Some sessions can have lots of patient sessions (community clinics can
sometimes have upwards of 50,000 patients in) and preloading all of this
can be slow and require a large amount of memory.

Often at this scale, it seems to be more performant to not avoid N+1
queries, particularly depending on the page that is trying to be loaded.

Author: thomasleese

app/components/app_session_actions_component.rb

@@ -6,11 +6,10 @@ class AppSessionActionsComponent < ViewComponent::Base
     <%= govuk_summary_list(rows:) %>
   ERB
 
-  def initialize(session, patient_sessions:)
+  def initialize(session)
     super
 
     @session = session
-    @patient_sessions = patient_sessions
   end
 
   def render?
@@ -19,7 +18,9 @@ def render?
 
   private
 
-  attr_reader :session, :patient_sessions
+  attr_reader :session
+
+  delegate :patient_sessions, to: :session
 
   def rows
     @rows ||= [

app/components/app_session_details_summary_component.rb

@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 
 class AppSessionDetailsSummaryComponent < ViewComponent::Base
-  def initialize(session, patient_sessions:)
+  def initialize(session)
     super
 
     @session = session
-    @patient_sessions = patient_sessions
   end
 
   def call
@@ -14,7 +13,9 @@ def call
 
   private
 
-  attr_reader :session, :patient_sessions
+  attr_reader :session
+
+  delegate :patient_sessions, to: :session
 
   def cohort_row
     count = patient_sessions.count

app/controllers/programmes_controller.rb

@@ -32,17 +32,7 @@ def sessions
       policy_scope(Session)
         .has_programme(@programme)
         .for_current_academic_year
-        .eager_load(:location)
-        .preload(
-          :session_dates,
-          patient_sessions: [
-            :gillick_assessments,
-            :session_attendances,
-            {
-              patient: %i[consent_statuses triage_statuses vaccination_records]
-            }
-          ]
-        )
+        .includes(:location, :session_dates)
         .order("locations.name")
   end
 

app/controllers/sessions_controller.rb

@@ -29,15 +29,7 @@ def completed
 
   def show
     respond_to do |format|
-      format.html do
-        @patient_sessions =
-          @session
-            .patient_sessions
-            .includes(:patient)
-            .in_programmes(@session.programmes)
-
-        render layout: "full"
-      end
+      format.html { render layout: "full" }
 
       format.xlsx do
         filename =

app/views/sessions/show.html.erb

@@ -27,14 +27,14 @@
 
       <h3 class="nhsuk-heading-s nhsuk-u-margin-bottom-2">Session details</h3>
 
-      <%= render AppSessionDetailsSummaryComponent.new(@session, patient_sessions: @patient_sessions) %>
+      <%= render AppSessionDetailsSummaryComponent.new(@session) %>
 
       <% if @session.unscheduled? %>
         <% if policy(@session).edit? %>
           <%= govuk_button_link_to "Schedule sessions", edit_session_path(@session), class: "app-button--secondary" %>
         <% end %>
       <% else %>
-        <%= render AppSessionActionsComponent.new(@session, patient_sessions: @patient_sessions) %>
+        <%= render AppSessionActionsComponent.new(@session) %>
 
         <div class="app-button-group">
           <% if policy(@session).edit? %>

spec/components/app_session_actions_component_spec.rb

@@ -3,11 +3,10 @@
 describe AppSessionActionsComponent do
   subject { render_inline(component) }
 
-  let(:component) { described_class.new(session, patient_sessions:) }
+  let(:component) { described_class.new(session) }
 
   let(:programmes) { [create(:programme, :hpv)] }
   let(:session) { create(:session, programmes:) }
-  let(:patient_sessions) { session.patient_sessions }
 
   before do
     create(

spec/components/app_session_details_summary_component_spec.rb

@@ -3,11 +3,10 @@
 describe AppSessionDetailsSummaryComponent do
   subject { render_inline(component) }
 
-  let(:component) { described_class.new(session, patient_sessions:) }
+  let(:component) { described_class.new(session) }
 
   let(:programme) { create(:programme, :hpv) }
   let(:session) { create(:session, programmes: [programme]) }
-  let(:patient_sessions) { session.patient_sessions }
 
   it { should have_text("Cohort\nNo children") }
   it { should have_text("Consent refused\nNo children") }