Prevent induction period start dates before Sept 2021

There's a chance we'll want to allow some earlier dates when importing
so maybe we should relax this rule for the migration.

app/models/induction_period.rb

@@ -21,6 +21,7 @@ class InductionPeriod < ApplicationRecord
             inclusion: { in: %w[fip cip diy],
                          message: "Choose an induction programme" }
 
+  validate :started_on_from_september_2021_onwards, if: -> { started_on.present? }
   validate :started_on_not_in_future, if: -> { started_on.present? }
   validate :finished_on_not_in_future, if: -> { finished_on.present? }
   validate :start_date_after_qts_date
@@ -34,6 +35,12 @@ class InductionPeriod < ApplicationRecord
 
 private
 
+  def started_on_from_september_2021_onwards
+    return if started_on >= Date.new(2021, 9, 1)
+
+    errors.add(:started_on, "Enter a start date after 1 September 2021")
+  end
+
   def started_on_not_in_future
     return if started_on <= Date.current
 

spec/components/teachers/past_induction_periods_component_spec.rb

@@ -52,15 +52,15 @@
       let!(:older_period) do
         FactoryBot.create(:induction_period,
                           teacher:,
-                          started_on: 4.years.ago,
-                          finished_on: 3.years.ago)
+                          started_on: 3.years.ago,
+                          finished_on: 2.years.ago)
       end
 
       it "displays all past periods in chronological order" do
         render_inline(component)
         dates = page.all(".govuk-summary-list__value").map(&:text)
-        expect(dates).to include(4.years.ago.to_date.to_fs(:govuk))
         expect(dates).to include(3.years.ago.to_date.to_fs(:govuk))
+        expect(dates).to include(2.years.ago.to_date.to_fs(:govuk))
       end
     end
   end

spec/models/induction_period_spec.rb

@@ -39,6 +39,21 @@
 
     it { is_expected.to validate_inclusion_of(:induction_programme).in_array(%w[fip cip diy]).with_message("Choose an induction programme") }
 
+    describe '#started_on_from_september_2021_onwards' do
+      context 'when started_on before September 2021' do
+        subject { FactoryBot.build(:induction_period, started_on:) }
+        let(:started_on) { Date.new(2021, 8, 31) }
+        before { subject.valid? }
+
+        it 'has a suitable error message' do
+          expect(subject.errors.messages[:started_on]).to include("Enter a start date after 1 September 2021")
+        end
+      end
+
+      it { is_expected.not_to allow_values(Date.new(2021, 8, 31)).for(:started_on) }
+      it { is_expected.to allow_values(Date.new(2021, 9, 1), Date.new(2021, 9, 2)).for(:started_on) }
+    end
+
     describe "started_on_not_in_future" do
       let(:induction_period) { FactoryBot.create(:induction_period, finished_on: nil) }