Add uuid validator

Validates the uuid format. Uses basic regex, so doesn't:
1) Check if there is a matching resource
2) Check the the format matches the uuid standard precisely.

app/validators/uuid_validator.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# UuidValidator
+# Validates that the supplied value is a valid uuid
+# by regular expression. This is a little more permissive
+# than strictly governed by the UUID format, but we're more
+# likely to see technically 'valid' uuids, which just don't
+# map to a resource, so lets keep things simple.
+class UuidValidator < ActiveModel::EachValidator
+  UUID = /\A[0-f]{8}-([0-f]{4}-){3}[0-f]{12}\z/.freeze
+
+  def validate_each(record, attribute, value)
+    return if UUID.match?(value)
+
+    record.errors.add(attribute, :uuid)
+  end
+end

config/locales/en.yml

@@ -55,6 +55,8 @@ en:
             group_id:
               duplicated_in_tag_set: must only appear once within a set
   errors:
+    messages:
+      uuid: is not a valid uuid
     attributes:
       cost_code:
         blank: must be present

spec/validators/uuid_validator_spec.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe UuidValidator do
+  describe '#validate_each' do
+    let(:record_class) { Ont::Request }
+    let(:record) { record_class.new }
+    let(:attribute) { :external_study_id }
+
+    before do
+      described_class.new(attributes: [attribute]).validate_each(record, attribute, value)
+    end
+
+    context 'with a nil value' do
+      let(:value) { nil }
+
+      it 'adds an error to the record' do
+        expect(record.errors.full_messages).to include('External study is not a valid uuid')
+      end
+    end
+
+    context 'with a non-uuid' do
+      let(:value) { 'not-a-uuid' }
+
+      it 'adds an error to the record' do
+        expect(record.errors.full_messages).to include('External study is not a valid uuid')
+      end
+    end
+
+    context 'with a uuid' do
+      let(:value) { generate(:uuid) }
+
+      it 'adds an error to the record' do
+        expect(record.errors.full_messages).to be_empty
+      end
+    end
+  end
+end