Add uniqueness constraint on applications.urn

Urn.call returns a unique urn for the application route.
URN suffix is generated from a UUID and guaranties uniqueness and randomness

.rubocop.yml

@@ -55,6 +55,10 @@ Rails/HasManyOrHasOneDependent:
   Exclude:
     - 'app/models/application.rb'
 
+Rails/UniqueValidationWithoutIndex:
+  Exclude:
+    - 'app/models/application.rb'
+
 Performance/MethodObjectAsBlock:
   Exclude:
     - 'app/models/summary.rb'

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

@@ -1,38 +1,97 @@
 # frozen_string_literal: true
 
 # Urn represents a Uniform Resource Name (URN) generator.
-# It generates a URN with a fixed prefix and a random alphanumeric suffix.
+# It generates a URN with a fixed prefix and suffix created from UUID
+# converted to based 8 to only contain digits.
 #
+# Suffix Algo base 8:
+# generate a UUID
+# remove any '-' chars
+# convert to a binary string
+# split in groups of 3 bits, each group is a one char in base 8; from "000" to "111"
+# convert each group to a char in base 10; from "0" to "7" => CHARSET
+# map each char to the CHARSET
 #
-# Example:
+# original idea: https://www.fastruby.io/blog/ruby/uuid/friendlier-uuid-urls-in-ruby.html
 #
-#   Urn.generate('teacher')          # => "IRPTE12345"
-#   Urn.generate('teacher')          # => "IRPTE12345"
-#   Urn.generate('salaried_trainee') # => "IRPST12345"
+# Example:
 #
+# Urn.call("teacher", base: 4)  => "IRPTE2332300320121012033313101030130020311212023302313031301300020132"
+# Urn.call("teacher", base: 8)  => "IRPTE1534743322003655447026533317435036675213160"
+# Urn.call("teacher", base: 16) => "IRPTEA8B252DB88114FCB991BF6763262CD04"
+# Urn.call("teacher", base: 32) => "IRPTE13MVE7PCVHKP5S5BQDC47KK2IQ"
+# Urn.call("teacher", base: 64) => "IRPTEBJ_8UdgBaxHZo9QRxawrn
+
 class Urn
-  attr_reader :value
-  attr_writer :suffix
+  CHARSET = %w[
+    0 1 2 3 4 5 6 7
+    8 9 A B C D E F
+    G H I J K L M N O P Q R S T U V
+    W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z - _
+  ].freeze
 
-  def self.generate(applicant_type)
-    code = applicant_type_code(applicant_type)
-    PREFIX + code + Array.new(LENGTH) { CHARSET.sample }.join
+  PREFIX = "IRP"
+  TEACHER_ROUTE = "teacher"
+  TRAINEE_ROUTE = "salaried_trainee"
+  ROUTE_MAPPING = {
+    TEACHER_ROUTE => "TE",
+    TRAINEE_ROUTE => "ST",
+  }.freeze
+
+  def self.call(...)
+    service = new(...)
+    service.generate_suffix
+    service.urn
   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"
-      "TE"
-    when "salaried_trainee"
-      "ST"
-    else
-      raise(ArgumentError, "Invalid applicant type: #{applicant_type}")
-    end
-  end
-  private_methods :applicant_type_code
+  def initialize(route, prefix: PREFIX, route_mapping: ROUTE_MAPPING, base: 64)
+    @prefix = prefix
+    @code = route_mapping.fetch(route)
+    @base = base
+    @bit_group_size = (base - 1).to_s(2).size
+  end
+
+  def urn
+    [prefix, code, suffix].join
+  end
+
+  def generate_suffix
+    @suffix = (
+      method(:binary_coded_decimal) >>
+      method(:add_padding) >>
+      method(:to_base) >>
+      method(:charset_encode)
+    ).call(compact_uuid)
+  end
+
+private
+
+  attr_reader :prefix, :code, :suffix, :base, :bit_group_size
+
+  def compact_uuid
+    SecureRandom.uuid.tr("-", "")
+  end
+
+  def binary_coded_decimal(uuid)
+    uuid.chars.map { |c| c.hex.to_s(2).rjust(4, "0") }.join
+  end
+
+  def add_padding(str)
+    missing_chars = str.size % bit_group_size
+    return str if missing_chars.zero?
+
+    padding = "0" * missing_chars
+    padding + str
+  end
+
+  def to_base(str)
+    regex = %r(.{#{bit_group_size}})
+    str
+      .scan(regex)
+      .map { |x| x.to_i(2) }
+  end
+
+  def charset_encode(str)
+    str.map { |x| CHARSET.fetch(x) }
+  end
 end

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.call(form.application_route),
       visa_type: form.visa_type,
     )
   end
@@ -100,10 +104,12 @@ 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
+
+  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/factories/applications.rb

@@ -31,7 +31,7 @@
     visa_type { VisaStep::VALID_ANSWERS_OPTIONS.reject { _1 == "Other" }.sample }
     date_of_entry { Time.zone.today }
     start_date { 1.month.from_now.to_date }
-    urn { Urn.generate(application_route) }
+    urn { Urn.call(application_route) }
 
     factory :teacher_application do
       application_route { "teacher" }

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

@@ -3,36 +3,36 @@
 require "rails_helper"
 
 RSpec.describe Urn do
-  subject(:urn) { described_class.generate(applicant_type) }
+  describe ".call" do
+    subject(:urn) { described_class.call(application_route, base: 8) }
 
-  describe ".generate" do
     context 'when applicant type is "teacher"' do
-      let(:applicant_type) { "teacher" }
+      let(:application_route) { "teacher" }
 
       it "generates a URN with the correct prefix and suffix" do
-        expect(urn).to match(/^IRPTE[0-9]{5}$/)
+        expect(urn).to match(/^IRPTE[0-7]{43}$/)
       end
 
       it "generates a Urn with a suffix of only characters in the CHARSET" do
-        charset = %w[0 1 2 3 4 5 6 7 8 9]
+        charset = %w[0 1 2 3 4 5 6 7]
 
-        expect(urn[5..9].chars).to all(be_in(charset))
+        expect(urn[5..47].chars).to all(be_in(charset))
       end
     end
 
     context 'when applicant type is "salaried_trainee"' do
-      let(:applicant_type) { "salaried_trainee" }
+      let(:application_route) { "salaried_trainee" }
 
       it "generates a URN with the correct prefix and suffix" do
-        expect(urn).to match(/^IRPST[0-9]{5}$/)
+        expect(urn).to match(/^IRPST[0-7]{43}$/)
       end
     end
 
     context "when an invalid applicant type is provided" do
-      let(:applicant_type) { "invalid_type" }
+      let(:application_route) { "invalid_type" }
 
       it "raises an ArgumentError" do
-        expect { urn }.to raise_error(ArgumentError, "Invalid applicant type: invalid_type")
+        expect { urn }.to raise_error(KeyError, 'key not found: "invalid_type"')
       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(:call).and_return(urn)
         end
 
         let(:urn) { "SOMEURN" }