guarantee that `klass` is not nil inside the preloader objects

activerecord/lib/active_record/associations/preloader.rb

@@ -133,7 +133,7 @@ def preloaders_for_hash(association, records, scope)
       def preloaders_for_one(association, records, scope)
         grouped_records(association, records).flat_map do |reflection, klasses|
           klasses.map do |rhs_klass, rs|
-            loader = preloader_for(reflection, rs).new(rhs_klass, rs, reflection, scope)
+            loader = preloader_for(reflection, rs, rhs_klass).new(rhs_klass, rs, reflection, scope)
             loader.run self
             loader
           end
@@ -173,7 +173,7 @@ def association_klass(reflection, record)
         end
       end
 
-      class NullPreloader
+      class AlreadyLoaded
         attr_reader :owners, :reflection
 
         def initialize(klass, owners, reflection, preload_scope)
@@ -188,9 +188,16 @@ def preloaded_records
         end
       end
 
-      def preloader_for(reflection, owners)
+      class NullPreloader
+        def self.new(klass, owners, reflection, preload_scope); self; end
+        def self.run(preloader); end
+      end
+
+      def preloader_for(reflection, owners, rhs_klass)
+        return NullPreloader unless rhs_klass
+
         if owners.first.association(reflection.name).loaded?
-          return NullPreloader
+          return AlreadyLoaded
         end
 
         case reflection.macro

activerecord/lib/active_record/associations/preloader/association.rb

@@ -86,7 +86,7 @@ def associated_records_by_owner(preloader)
             h[owner] = []
           end
 
-          if klass && owner_keys.any?
+          if owner_keys.any?
             # Some databases impose a limit on the number of ids in a list (in Oracle it's 1000)
             # Make several smaller queries if necessary or make one query if the adapter supports it
             sliced  = owner_keys.each_slice(klass.connection.in_clause_length || owner_keys.size)