Reduce SQL queries in the study groups controller

Fixes CODEOCEAN-Y7

app/controllers/study_groups_controller.rb

@@ -7,20 +7,23 @@ class StudyGroupsController < ApplicationController
 
   def index
     @search = policy_scope(StudyGroup).ransack(params[:q])
-    @study_groups = @search.result.includes(:consumer).order(:name).paginate(page: params[:page], per_page: per_page_param)
+    @study_groups_paginate = @search.result.includes(:consumer).order(:name).paginate(page: params[:page], per_page: per_page_param)
+    @study_groups = @study_groups_paginate.joins(:study_group_memberships)
+      .group(:id, :name, :external_id, :consumer_id, :created_at, :updated_at)
+      .select(:id, :name, :external_id, :consumer_id, :created_at, :updated_at, 'count(study_group_memberships.id) as user_count')
     authorize!
   end
 
   def show; end
 
   def edit
-    @members = StudyGroupMembership.where(user: @study_group.users, study_group: @study_group).includes(:user)
+    @members = @study_group.study_group_memberships.includes(:user)
   end
 
   def update
     myparams = study_group_params
     myparams[:external_users] =
-      StudyGroupMembership.find(myparams[:study_group_membership_ids].compact_blank).map(&:user)
+      @study_group.study_group_memberships.where(id: myparams[:study_group_membership_ids].compact_blank).includes(:user).map(&:user)
     myparams.delete(:study_group_membership_ids)
     update_and_respond(object: @study_group, params: myparams)
   end

app/models/study_group.rb

@@ -19,7 +19,7 @@ class StudyGroup < ApplicationRecord
   end
 
   def user_count
-    study_group_memberships.distinct.count
+    attributes['user_count'] || study_group_memberships.size
   end
 
   def to_s

app/views/study_groups/index.html.slim

@@ -9,4 +9,4 @@ h1 = StudyGroup.model_name.human(count: 2)
     = f.search_field(:name_cont, class: 'form-control', placeholder: t('activerecord.attributes.study_group.name'))
 
 = render('table', study_groups: @study_groups)
-= render('shared/pagination', collection: @study_groups)
+= render('shared/pagination', collection: @study_groups_paginate)