# UNDF: UNDF-2026-000000241
Fixes rails-0001: ActiveRecord Preloader::Batch — future_tables Array#include? inside loaders.reject loop.

--- a/activerecord/lib/active_record/associations/preloader/batch.rb
+++ b/activerecord/lib/active_record/associations/preloader/batch.rb

@@ DEFECT rails-0001: line 20-24

 until branches.empty?
     future_tables = branches.flat_map do |branch|
         branch.future_classes - branch.runnable_loaders.map(&:klass)
-    end.map(&:table_name).uniq                                           # Array — O(F) include?
+    end.map(&:table_name).to_set                                         # FIX: Set for O(1) include?

     target_loaders = loaders.reject { |l| future_tables.include?(l.table_name) }   # O(L) × O(1) — fixed

# BEFORE: future_tables is Array; loaders.reject calls Array#include? → O(L×F) per batch round-trip
# AFTER:  future_tables is Set;   loaders.reject calls Set#include?   → O(L) per batch round-trip
# With D-depth association trees: O(D×L×F) → O(D×L)