Merge pull request rails#51507 from joshuay03/fix-polymorphic-has-many-through-with-association-scope

[Fix rails#40109] Incorrect joins for polymorphic `has_many through:` with association scope

Author: kamipo

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix association scopes applying on the incorrect join when using a polymorphic `has_many through:`.
+
+    *Joshua Young*
+
 *   Allow `ActiveRecord::Base#pluck` to accept hash arguments with symbol and string values.
 
     ```ruby

activerecord/lib/active_record/reflection.rb

@@ -1236,7 +1236,10 @@ def initialize(reflection, previous_reflection)
       end
 
       def join_scopes(table, predicate_builder = nil, klass = self.klass, record = nil) # :nodoc:
-        scopes = @previous_reflection.join_scopes(table, predicate_builder, klass, record) + super
+        scopes = super
+        unless @previous_reflection.through_reflection?
+          scopes += @previous_reflection.join_scopes(table, predicate_builder, klass, record)
+        end
         scopes << build_scope(table, predicate_builder, klass).instance_exec(record, &source_type_scope)
       end
 

activerecord/test/cases/associations/nested_through_associations_test.rb

@@ -639,6 +639,19 @@ def test_polymorphic_has_many_through_joined_different_table_twice
     assert_equal hotel, Hotel.joins(:cake_designers, :drink_designers).take
   end
 
+  def test_has_many_through_polymorphic_with_scope
+    Post.delete_all
+
+    post = Post.create!(title: "Catchy Title", body: "Interesting body.")
+    category = Category.create!(name: "Anything")
+    Post::CategoryPost.create!(post: post, category: category)
+
+    author = authors(:bob)
+    Essay.create!(writer: author, category: category)
+
+    assert_equal 1, Post.joins(:authors_of_essays_named_bob).count
+  end
+
   def test_has_many_through_reset_source_reflection_after_loading_is_complete
     preloaded = Category.preload(:ordered_post_comments).find(1, 2).last
     original = Category.find(2)

activerecord/test/models/post.rb

@@ -124,6 +124,13 @@ def greeting
   has_and_belongs_to_many :categories
   has_and_belongs_to_many :special_categories, join_table: "categories_posts", association_foreign_key: "category_id"
 
+  has_many :essays, through: :categories
+  has_many :authors_of_essays_named_bob,
+    -> { where(name: "Bob") },
+    through: :essays,
+    source: :writer,
+    source_type: "Author"
+
   has_many :taggings, as: :taggable, counter_cache: :tags_count
   has_many :tags, through: :taggings do
     def add_joins_and_select