Updated preference init to skip redundant write

Persistable stores Preferable preferences in a YAML-serialized column.
The initialize_preference_defaults callback ran on every load and
reassigned the preferences attribute even when the merged defaults
already matched the stored value. That assignment switches the attribute
to ActiveModel's "from user" state, whose reads round-trip the hash
through YAML on every access, while "from database" reads only load it.

Hot paths that read calculator preferences per record paid this on every
request. Shipping-rate estimation, for example, reads each shipping
method's calculator preferences and re-dumped YAML once per method per
estimation.

Guard the assignment so it only runs when the merge actually changes the
stored preferences. Unchanged records stay on the cheaper read path and
defaults are still backfilled when missing.

Author: ikraamg

core/app/models/concerns/spree/preferences/persistable.rb

@@ -20,9 +20,10 @@ module Persistable
       private
 
       def initialize_preference_defaults
-        if has_attribute?(:preferences)
-          self.preferences = default_preferences.merge(preferences)
-        end
+        return unless has_attribute?(:preferences)
+
+        merged = default_preferences.merge(preferences)
+        self.preferences = merged unless merged == preferences
       end
     end
   end

core/spec/models/spree/preferences/preferable_spec.rb

@@ -387,6 +387,17 @@ class ComplexPreferableClass
       end
     end
 
+    it "does not re-serialize unchanged preferences when read after load" do
+      loaded = PrefTest.find(@pt.id)
+
+      # NOTE: assigning preferences in after_initialize (even an identical value) flips
+      # the attribute to "from user" state, so every later read re-dumps YAML. Skipping
+      # the redundant assignment keeps reads on the cheaper "from database" path.
+      expect(Psych).not_to receive(:safe_dump)
+
+      loaded.preferences
+    end
+
     it "clear preferences when record is deleted" do
       @pt.save!
       @pt.preferred_pref_test_pref = "lmn"