diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index d31f7432cd..0cc836f991 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -77,7 +77,7 @@ module ActiveRecord
             # 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)
-            records = sliced.map { |slice| records_for(slice).to_a }.flatten
+            records = sliced.flat_map { |slice| records_for(slice).to_a }
           end
 
           # Each record may have multiple owners, and vice-versa