Add uniqueness constraint on applications.urn

Urn.next_available method returns a unique urn based the existing ones
for the application route.

Over time finding a unique unused urn will be a longer operation

app/models/application.rb

@@ -67,4 +67,5 @@ def mark_as_qa!
   validates(:subject, presence: true)
   validates(:visa_type, presence: true)
   validates(:applicant, presence: true)
+  validates(:urn, presence: true, uniqueness: true)
 end

app/models/urn.rb

@@ -11,19 +11,40 @@
 #   Urn.generate('salaried_trainee') # => "IRPST12345"
 #
 class Urn
+  class GenerationError < StandardError; end
+
+  CHARSET = %w[0 1 2 3 4 5 6 7 8 9].freeze
+  PREFIX = "IRP"
+  LENGTH = 5
+
+  MAX_URN_SET_PER_ROUTE = ("9" * LENGTH).to_i
+  INVALID_SUFFIX = "0" * LENGTH
+
+  private_constant :CHARSET, :PREFIX, :LENGTH
+
   attr_reader :value
   attr_writer :suffix
 
+  def self.next_available(route, used_urns)
+    raise(GenerationError, "No more urn available for route #{route}") if used_urns.size >= MAX_URN_SET_PER_ROUTE
+
+    urn = nil
+
+    loop do
+      urn = generate(route)
+
+      next if urn.include?(INVALID_SUFFIX)
+      break unless used_urns.include?(urn)
+    end
+
+    urn
+  end
+
   def self.generate(applicant_type)
     code = applicant_type_code(applicant_type)
     PREFIX + code + Array.new(LENGTH) { CHARSET.sample }.join
   end
 
-  CHARSET = %w[0 1 2 3 4 5 6 7 8 9].freeze
-  PREFIX = "IRP"
-  LENGTH = 5
-  private_constant :CHARSET, :PREFIX, :LENGTH
-
   def self.applicant_type_code(applicant_type)
     case applicant_type
     when "teacher"

app/services/submit_form.rb

@@ -11,8 +11,9 @@ def initialize(form, ip_address)
     @form = form
     @ip_address = ip_address
     @success = false
+    @counter = 0
   end
-  attr_reader :form, :ip_address, :application
+  attr_reader :form, :ip_address, :application, :counter
 
   delegate :errors, to: :form
 
@@ -30,6 +31,9 @@ def submit_form!
     create_application_records
     send_applicant_email
     @success = true
+  rescue StandardError => e
+    Sentry.capture_exception(e)
+    errors.add(:base, :technical_error, message: "We are unable to submit your application at the moment. Please try again later")
   end
 
 private
@@ -75,20 +79,20 @@ def create_applicant
         city: form.city,
         postcode: form.postcode,
       },
-      school: @school,
+      school: school,
     )
   end
 
   def create_application
     @application = Application.create!(
-      applicant: @applicant,
+      applicant: applicant,
       application_date: Date.current.to_s,
       application_route: form.application_route,
       application_progress: ApplicationProgress.new,
       date_of_entry: form.date_of_entry,
       start_date: form.start_date,
       subject: SubjectStep.new(form).answer.formatted_value,
-      urn: Urn.generate(form.application_route),
+      urn: Urn.next_available(form.application_route, used_urns),
       visa_type: form.visa_type,
     )
   end
@@ -100,10 +104,18 @@ def delete_form
   def send_applicant_email
     GovukNotifyMailer
       .with(
-        email: @applicant.email_address,
+        email: applicant.email_address,
         urn: application.urn,
       )
       .application_submission
       .deliver_later
   end
+
+  def used_urns
+    Application
+      .where(application_route: form.application_route)
+      .pluck(:urn)
+  end
+
+  attr_reader :school, :applicant
 end

db/migrate/20230919103917_add_uniqueness_to_application_urn.rb

@@ -0,0 +1,9 @@
+class AddUniquenessToApplicationUrn < ActiveRecord::Migration[7.0]
+  def up
+    drop_view :duplicate_applications
+    add_index(:applications, :urn)
+    create_view :duplicate_applications, version: 3
+  end
+
+  def down; end
+end

spec/models/application_spec.rb

@@ -35,6 +35,8 @@
       it { expect(application).to validate_presence_of(:subject) }
       it { expect(application).to validate_presence_of(:visa_type) }
       it { expect(application).to validate_presence_of(:applicant) }
+      it { expect(application).to validate_presence_of(:urn) }
+      it { expect(application).to validate_uniqueness_of(:urn) }
     end
   end
 

spec/models/urn_spec.rb

@@ -3,9 +3,9 @@
 require "rails_helper"
 
 RSpec.describe Urn do
-  subject(:urn) { described_class.generate(applicant_type) }
-
   describe ".generate" do
+    subject(:urn) { described_class.generate(applicant_type) }
+
     context 'when applicant type is "teacher"' do
       let(:applicant_type) { "teacher" }
 
@@ -36,4 +36,34 @@
       end
     end
   end
+
+  describe ".next_available" do
+    subject(:urn) { described_class.next_available(application_route, used_urns) }
+
+    let(:application_route) { "teacher" }
+    let(:used_urns) { %w[IRPTE45632 IRPTE74125] }
+
+    context "when all urns possible have been used" do
+      let(:used_urns) { Array.new(Urn::MAX_URN_SET_PER_ROUTE) }
+
+      it "raises an GenerationError" do
+        expect { urn }.to raise_error(Urn::GenerationError, "No more urn available for route #{application_route}")
+      end
+    end
+
+    context "never returns an urn with only 0 in suffix" do
+      before do
+        allow(described_class).to receive(:generate).and_return(invalid_urn, valid_urn)
+      end
+
+      let(:invalid_urn) { "IRPTE00000" }
+      let(:valid_urn) { "IRPTE22222" }
+
+      it { expect(urn).to eq(valid_urn) }
+    end
+
+    context "returns an unused urn" do
+      it { expect(urn).to include("IRPTE") }
+    end
+  end
 end

spec/services/submit_form_spec.rb

@@ -133,7 +133,7 @@
 
       context "applicant email" do
         before do
-          allow(Urn).to receive(:generate).and_return(urn)
+          allow(Urn).to receive(:next_available).and_return(urn)
         end
 
         let(:urn) { "SOMEURN" }