Fix preloading for hmt relations with conditions

Author: apauly

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fix regression bug that caused ignoring additional conditions for preloading has_many-through relations.
+
+    Fixes #43132
+
+    *Alexander Pauly*
+
 *   Fix `has_many` inversing recursion on models with recursive associations.
 
     *Gannon McGibbon*

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

@@ -263,7 +263,7 @@ def owner_key_type
           end
 
           def reflection_scope
-            @reflection_scope ||= reflection.join_scopes(klass.arel_table, klass.predicate_builder, klass).inject(&:merge!) || klass.unscoped
+            @reflection_scope ||= reflection.join_scopes(klass.arel_table, klass.predicate_builder, klass).inject(klass.unscoped, &:merge!)
           end
 
           def build_scope

activerecord/test/cases/associations_test.rb

@@ -428,6 +428,18 @@ def test_preload_makes_correct_number_of_queries_on_relation
     end
   end
 
+  def test_preload_for_hmt_with_conditions
+    post = posts(:welcome)
+    _normal_category = post.categories.create!(name: "Normal")
+    special_category = post.special_categories.create!(name: "Special")
+
+    preloader = ActiveRecord::Associations::Preloader.new(records: [post], associations: :hmt_special_categories)
+    preloader.call
+
+    assert_equal 1, post.hmt_special_categories.length
+    assert_equal [special_category], post.hmt_special_categories
+  end
+
   def test_preload_groups_queries_with_same_scope
     book = books(:awdr)
     post = posts(:welcome)

activerecord/test/models/post.rb

@@ -114,6 +114,7 @@ def greeting
 
   has_many :category_posts, class_name: "CategoryPost"
   has_many :scategories, through: :category_posts, source: :category
+  has_many :hmt_special_categories, -> { where.not(name: nil) },  through: :category_posts, source: :category, class_name: "SpecialCategory"
   has_and_belongs_to_many :categories
   has_and_belongs_to_many :special_categories, join_table: "categories_posts", association_foreign_key: "category_id"