has_many :through with unscope should affect to through scope

The order of scope evaluation should be from through scope to the
association's own scope. Otherwise the association's scope cannot affect
to through scope.

Fixes #13677.
Closes #28449.

activerecord/lib/active_record/associations/association_scope.rb

@@ -24,10 +24,10 @@ def scope(association)
         scope = klass.unscoped
         owner = association.owner
         alias_tracker = AliasTracker.create(klass.connection, klass.table_name)
-        chain_head, chain_tail = get_chain(reflection, association, alias_tracker)
+        chain = get_chain(reflection, association, alias_tracker)
 
         scope.extending! reflection.extensions
-        add_constraints(scope, owner, chain_head, chain_tail)
+        add_constraints(scope, owner, chain)
       end
 
       def join_type
@@ -98,7 +98,6 @@ def next_chain_scope(scope, table, reflection, foreign_table, next_reflection)
         end
 
         class ReflectionProxy < SimpleDelegator # :nodoc:
-          attr_accessor :next
           attr_reader :alias_name
 
           def initialize(reflection, alias_name)
@@ -111,36 +110,32 @@ def all_includes; nil; end
 
         def get_chain(reflection, association, tracker)
           name = reflection.name
-          runtime_reflection = Reflection::RuntimeReflection.new(reflection, association)
-          previous_reflection = runtime_reflection
+          chain = [Reflection::RuntimeReflection.new(reflection, association)]
           reflection.chain.drop(1).each do |refl|
             alias_name = tracker.aliased_table_for(
               refl.table_name,
               refl.alias_candidate(name),
               refl.klass.type_caster
             )
-            proxy = ReflectionProxy.new(refl, alias_name)
-            previous_reflection.next = proxy
-            previous_reflection = proxy
+            chain << ReflectionProxy.new(refl, alias_name)
           end
-          [runtime_reflection, previous_reflection]
+          chain
         end
 
-        def add_constraints(scope, owner, chain_head, chain_tail)
-          owner_reflection = chain_tail
+        def add_constraints(scope, owner, chain)
+          owner_reflection = chain.last
           table = owner_reflection.alias_name
           scope = last_chain_scope(scope, table, owner_reflection, owner)
 
-          reflection = chain_head
-          while reflection
+          chain.each_cons(2) do |reflection, next_reflection|
             table = reflection.alias_name
-            next_reflection = reflection.next
-
-            unless reflection == chain_tail
-              foreign_table = next_reflection.alias_name
-              scope = next_chain_scope(scope, table, reflection, foreign_table, next_reflection)
-            end
+            foreign_table = next_reflection.alias_name
+            scope = next_chain_scope(scope, table, reflection, foreign_table, next_reflection)
+          end
 
+          chain_head = chain.first
+          chain.reverse_each do |reflection|
+            table = reflection.alias_name
             # Exclude the scope of the association itself, because that
             # was already merged in the #scope method.
             reflection.constraints.each do |scope_chain_item|
@@ -158,8 +153,6 @@ def add_constraints(scope, owner, chain_head, chain_tail)
               scope.where_clause += item.where_clause
               scope.order_values |= item.order_values
             end
-
-            reflection = next_reflection
           end
 
           scope

activerecord/lib/active_record/reflection.rb

@@ -1077,8 +1077,6 @@ def source_type_info
     end
 
     class RuntimeReflection < PolymorphicReflection # :nodoc:
-      attr_accessor :next
-
       def initialize(reflection, association)
         @reflection = reflection
         @association = association

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1250,6 +1250,10 @@ def test_has_many_through_do_not_cache_association_reader_if_the_though_method_h
     TenantMembership.current_member = nil
   end
 
+  def test_has_many_through_with_unscope_should_affect_to_through_scope
+    assert_equal [comments(:eager_other_comment1)], authors(:mary).unordered_comments
+  end
+
   def test_has_many_through_with_scope_should_respect_table_alias
     family = Family.create!
     users = 3.times.map { User.create! }

activerecord/test/models/author.rb

@@ -40,6 +40,7 @@ def ratings
            class_name: "Post"
 
   has_many :comments_desc, -> { order("comments.id DESC") }, through: :posts, source: :comments
+  has_many :unordered_comments, -> { unscope(:order).distinct }, through: :posts_sorted_by_id_limited, source: :comments
   has_many :funky_comments, through: :posts, source: :comments
   has_many :ordered_uniq_comments, -> { distinct.order("comments.id") }, through: :posts, source: :comments
   has_many :ordered_uniq_comments_desc, -> { distinct.order("comments.id DESC") }, through: :posts, source: :comments