Perf (#592)

* Memoize start year

* Reset

* Recalculate rather than reset

app/models/case_log.rb

@@ -22,6 +22,7 @@ class CaseLog < ApplicationRecord
   has_paper_trail
 
   validates_with CaseLogValidator
+  before_validation :recalculate_start_year!, if: :startdate_changed?
   before_validation :process_postcode_changes!, if: :postcode_full_changed?
   before_validation :process_previous_postcode_changes!, if: :ppostcode_full_changed?
   before_validation :reset_invalidated_dependent_fields!
@@ -66,10 +67,16 @@ class CaseLog < ApplicationRecord
   end
 
   def collection_start_year
+    return @start_year if @start_year
     return unless startdate
 
     window_end_date = Time.zone.local(startdate.year, 4, 1)
-    startdate < window_end_date ? startdate.year - 1 : startdate.year
+    @start_year = startdate < window_end_date ? startdate.year - 1 : startdate.year
+  end
+
+  def recalculate_start_year!
+    @start_year = nil
+    collection_start_year
   end
 
   def form_name