add hard validation that percentage owned after a staircasing transaction cannot be less than the percentage bought in that transaction, with associated tests

app/models/form/sales/pages/staircase_bought_value_check.rb

@@ -13,8 +13,8 @@ class Form::Sales::Pages::StaircaseBoughtValueCheck < ::Form::Page
         {
           "key" => "stairbought",
           "i18n_template" => "percentage",
-        }
-      ]
+        },
+      ],
     }
     @informative_text = {}
   end

app/models/validations/sales/financial_validations.rb

@@ -27,4 +27,12 @@ module Validations::Sales::FinancialValidations
       record.errors.add :cashdis, I18n.t("validations.financial.cash_discount_invalid")
     end
   end
+
+  def validate_percentage_bought_not_greater_than_percentage_owned(record)
+    return unless record.stairbought && record.stairowned
+
+    if record.stairbought > record.stairowned
+      record.errors.add :stairowned, I18n.t("validations.financial.staircasing.percentage_bought_must_be_greater_than_percentage_owned")
+    end
+  end
 end

config/locales/en.yml

@@ -277,6 +277,9 @@ en:
         out_of_range: "Household rent and other charges must be between %{min_chcharge} and %{max_chcharge} if paying %{period}"
         not_provided: "Enter how much rent and other charges the household pays %{period}"
       cash_discount_invalid: "Cash discount must be £0 - £999,999"
+      staircasing:
+        percentage_bought_must_be_greater_than_percentage_owned: "Total percentage buyer now owns must be more than percentage bought in this transaction"
+
     household:
       reasonpref:
         not_homeless: "Answer cannot be ‘homeless or about to lose their home’ as the tenant was not homeless immediately prior to this letting"

spec/models/validations/sales/financial_validations_spec.rb

@@ -99,4 +99,31 @@ RSpec.describe Validations::Sales::FinancialValidations do
       expect(record.errors["cashdis"]).to be_empty
     end
   end
+
+  describe "#validate_percentage_bought_not_greater_than_percentage_owned" do
+    let(:record) { FactoryBot.create(:sales_log) }
+
+    it "does not add an error if the percentage bought is less than the percentage owned" do
+      record.stairbought = 20
+      record.stairowned = 40
+      financial_validator.validate_percentage_bought_not_greater_than_percentage_owned(record)
+      expect(record.errors["stairbought"]).to be_empty
+      expect(record.errors["stairowned"]).to be_empty
+    end
+
+    it "does not add an error if the percentage bought is equal to the percentage owned" do
+      record.stairbought = 30
+      record.stairowned = 30
+      financial_validator.validate_percentage_bought_not_greater_than_percentage_owned(record)
+      expect(record.errors["stairbought"]).to be_empty
+      expect(record.errors["stairowned"]).to be_empty
+    end
+
+    it "adds an error to stairowned and not stairbought if the percentage bought is more than the percentage owned" do
+      record.stairbought = 50
+      record.stairowned = 40
+      financial_validator.validate_percentage_bought_not_greater_than_percentage_owned(record)
+      expect(record.errors["stairowned"]).to include(match I18n.t("validations.financial.staircasing.percentage_bought_must_be_greater_than_percentage_owned"))
+    end
+  end
 end