set vacdays after setting the voiddate (#900)

app/models/derived_variables/lettings_log_variables.rb

@@ -58,7 +58,6 @@ module DerivedVariables::LettingsLogVariables
 
     self.hhtype = household_type
     self.new_old = new_or_existing_tenant
-    self.vacdays = property_vacant_days
 
     if is_supported_housing?
       if location
@@ -68,6 +67,7 @@ module DerivedVariables::LettingsLogVariables
         self.voiddate = startdate
       end
     end
+    self.vacdays = property_vacant_days
 
     set_housingneeds_fields if housingneeds?
   end

spec/models/lettings_log_spec.rb

@@ -1395,6 +1395,7 @@ RSpec.describe LettingsLog do
           renewal: 1,
           startdate: Time.zone.local(2021, 4, 10),
           created_at: Time.utc(2022, 2, 8, 16, 52, 15),
+          needstype: 2,
         })
       end
 
@@ -1431,6 +1432,12 @@ RSpec.describe LettingsLog do
         expect(record_from_db["referral"]).to eq(0)
         expect(lettings_log["referral"]).to eq(0)
       end
+
+      it "correctly derives and saves vacdays" do
+        record_from_db = ActiveRecord::Base.connection.execute("select vacdays from lettings_logs where id=#{lettings_log.id}").to_a[0]
+        expect(record_from_db["vacdays"]).to eq(0)
+        expect(lettings_log["vacdays"]).to eq(0)
+      end
     end
 
     context "when answering the household characteristics questions" do