if one eligible_itt_subject show better error msg

app/forms/journeys/additional_payments_for_teaching/eligible_itt_subject_form.rb

@@ -5,7 +5,30 @@ class EligibleIttSubjectForm < Form
 
       attribute :eligible_itt_subject, :string
 
-      validates :eligible_itt_subject, inclusion: {in: :available_options, message: i18n_error_message(:inclusion)}
+      validates :eligible_itt_subject,
+        inclusion: {
+          in: :available_options,
+          message: i18n_error_message(:inclusion)
+        },
+        unless: :boolean_answer?
+
+      validates :eligible_itt_subject,
+        inclusion: {
+          in: :available_options,
+          message: ->(object, _data) {
+            "Select yes if you did your #{qualification_to_substring(object.answers.qualification.to_sym)} in #{object.available_subjects.first}"
+          }
+        },
+        if: :boolean_answer?
+
+      def self.qualification_to_substring(qualification_symbol)
+        {
+          undergraduate_itt: "undergraduate initial teacher training (ITT)",
+          postgraduate_itt: "postgraduate initial teacher training (ITT)",
+          assessment_only: "assessment",
+          overseas_recognition: "teaching qualification"
+        }[qualification_symbol]
+      end
 
       def available_subjects
         @available_subjects ||= subject_symbols.map(&:to_s)
@@ -52,6 +75,10 @@ def save
       def eligible_itt_subject_changed?
         journey_session.answers.eligible_itt_subject != eligible_itt_subject
       end
+
+      def boolean_answer?
+        !available_subjects.many?
+      end
     end
   end
 end

spec/forms/journeys/additional_payments_for_teaching/eligible_itt_subject_form_spec.rb

@@ -31,7 +31,7 @@
 
   let(:itt_academic_year) { AcademicYear.new(2020) }
 
-  let(:form) do
+  subject(:form) do
     described_class.new(
       journey: journey,
       journey_session: journey_session,
@@ -40,15 +40,37 @@
   end
 
   describe "validations" do
-    subject { form }
-
     let(:params) { ActionController::Parameters.new }
 
     it do
       is_expected.to validate_inclusion_of(:eligible_itt_subject)
         .in_array(form.available_options)
         .with_message("Select a subject")
     end
+
+    context "when single subject available" do
+      before do
+        allow(JourneySubjectEligibilityChecker).to receive(:selectable_subject_symbols).and_return([:mathematics])
+      end
+
+      let(:answers) do
+        build(
+          :additional_payments_answers,
+          attributes_for(
+            :additional_payments_answers,
+            :with_qualification,
+            itt_academic_year: itt_academic_year,
+            current_school_id: create(:school, :early_career_payments_eligible).id
+          )
+        )
+      end
+
+      it "returns contextual error message" do
+        expect(subject).to validate_inclusion_of(:eligible_itt_subject)
+          .in_array(["mathematics"])
+          .with_message("Select yes if you did your postgraduate initial teacher training (ITT) in mathematics")
+      end
+    end
   end
 
   describe "#available_subjects" do