Fuse traversals of the record set when preloading

Although consuming code will almost certainly retraverse this set, we can avoid
walking it twice here. As an extra upside, we can avoid the double-use of an
identity-sensitive hash; this is convenient, because it is another collection we
actually don't need to build.

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

@@ -27,39 +27,41 @@ def run
         end
 
         def records_by_owner
-          # owners can be duplicated when a relation has a collection association join
-          # #compare_by_identity makes such owners different hash keys
-          @records_by_owner ||= preloaded_records.each_with_object({}.compare_by_identity) do |record, result|
-            owners_by_key[convert_key(record[association_key_name])].each do |owner|
-              (result[owner] ||= []) << record
-            end
-          end
+          load_records unless defined?(@records_by_owner)
+
+          @records_by_owner
         end
 
         def preloaded_records
-          return @preloaded_records if defined?(@preloaded_records)
+          load_records unless defined?(@preloaded_records)
+
+          @preloaded_records
+        end
+
+        private
+          attr_reader :owners, :reflection, :preload_scope, :model, :klass
 
-          raw_records = owner_keys.empty? ? [] : records_for(owner_keys)
-          seen_records_by_owner = {}.compare_by_identity
+          def load_records
+            # owners can be duplicated when a relation has a collection association join
+            # #compare_by_identity makes such owners different hash keys
+            @records_by_owner = {}.compare_by_identity
+            raw_records = owner_keys.empty? ? [] : records_for(owner_keys)
 
-          @preloaded_records = raw_records.select do |record|
-            assignments = []
+            @preloaded_records = raw_records.select do |record|
+              assignments = []
 
-            owners_by_key[convert_key(record[association_key_name])].each do |owner|
-              entries = (seen_records_by_owner[owner] ||= [])
+              owners_by_key[convert_key(record[association_key_name])].each do |owner|
+                entries = (@records_by_owner[owner] ||= [])
 
-              if reflection.collection? || entries.empty?
-                entries << record
-                assignments << record
+                if reflection.collection? || entries.empty?
+                  entries << record
+                  assignments << record
+                end
               end
-            end
 
-            !assignments.empty?
+              !assignments.empty?
+            end
           end
-        end
-
-        private
-          attr_reader :owners, :reflection, :preload_scope, :model, :klass
 
           # The name of the key on the associated records
           def association_key_name