Add foreign key on patient sessions

This adds a foreign key to patients and sessions from the patient
sessions table, ensuring that patient sessions always refer to patients
and sessions that exist.

It seems like this was missed when the table was set up originally.

app/models/patient_session.rb

@@ -15,6 +15,11 @@
 #  index_patient_sessions_on_patient_id_and_session_id  (patient_id,session_id) UNIQUE
 #  index_patient_sessions_on_session_id_and_patient_id  (session_id,patient_id) UNIQUE
 #
+# Foreign Keys
+#
+#  fk_rails_...  (patient_id => patients.id)
+#  fk_rails_...  (session_id => sessions.id)
+#
 
 class PatientSession < ApplicationRecord
   audited

db/migrate/20241213102755_add_foreign_key_on_patient_sessions.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class AddForeignKeyOnPatientSessions < ActiveRecord::Migration[8.0]
+  def change
+    add_foreign_key :patient_sessions, :patients
+    add_foreign_key :patient_sessions, :sessions
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2024_12_12_145341) do
+ActiveRecord::Schema[8.0].define(version: 2024_12_13_102755) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
   enable_extension "pg_trgm"
@@ -826,6 +826,8 @@
   add_foreign_key "organisation_programmes", "programmes"
   add_foreign_key "parent_relationships", "parents"
   add_foreign_key "parent_relationships", "patients"
+  add_foreign_key "patient_sessions", "patients"
+  add_foreign_key "patient_sessions", "sessions"
   add_foreign_key "patients", "cohorts"
   add_foreign_key "patients", "locations", column: "gp_practice_id"
   add_foreign_key "patients", "locations", column: "school_id"

spec/factories/patient_sessions.rb

@@ -15,6 +15,11 @@
 #  index_patient_sessions_on_patient_id_and_session_id  (patient_id,session_id) UNIQUE
 #  index_patient_sessions_on_session_id_and_patient_id  (session_id,patient_id) UNIQUE
 #
+# Foreign Keys
+#
+#  fk_rails_...  (patient_id => patients.id)
+#  fk_rails_...  (session_id => sessions.id)
+#
 FactoryBot.define do
   factory :patient_session do
     transient do

spec/models/patient_session_spec.rb

@@ -15,6 +15,11 @@
 #  index_patient_sessions_on_patient_id_and_session_id  (patient_id,session_id) UNIQUE
 #  index_patient_sessions_on_session_id_and_patient_id  (session_id,patient_id) UNIQUE
 #
+# Foreign Keys
+#
+#  fk_rails_...  (patient_id => patients.id)
+#  fk_rails_...  (session_id => sessions.id)
+#
 
 describe PatientSession do
   let(:programme) { create(:programme) }