Fix nested has many :through associations on unpersisted instances

Fixes: #16313

actionmailer/CHANGELOG.md

@@ -1,3 +1,39 @@
+*   Fix nested `has many :through` associations on unpersisted parent instances.
+
+    For example, if you have
+
+        class Post < ActiveRecord::Base
+          has_many :books, through: :author
+          has_many :subscriptions, through: :books
+        end
+
+        class Author < ActiveRecord::Base
+          has_one :post
+          has_many :books
+          has_many :subscriptions, through: :books
+        end
+
+        class Book < ActiveRecord::Base
+          belongs_to :author
+          has_many :subscriptions
+        end
+
+        class Subscription < ActiveRecord::Base
+          belongs_to :book
+        end
+
+    Before:
+        If `post` is not persisted, e.g `post = Post.new`, then `post.subscriptions`
+        will be empty no matter what.
+
+    After:
+        If `post` is not persisted, then `post.subscriptions` can be set and used
+        just like it would if `post` were persisted.
+
+    Fixes #16313.
+
+    *Zoltan Kiss*
+
 *   Add `assert_enqueued_emails` and `assert_no_enqueued_emails`.
 
     Example:

activerecord/lib/active_record/associations/through_association.rb

@@ -77,7 +77,7 @@ def stale_state
         end
 
         def foreign_key_present?
-          through_reflection.belongs_to? && !owner[through_reflection.foreign_key].nil?
+          through_reflection.belongs_to_or_through? && !owner[through_reflection.foreign_key].nil?
         end
 
         def ensure_mutable

activerecord/lib/active_record/reflection.rb

@@ -139,6 +139,10 @@ def primary_key_type
         klass.type_for_attribute(klass.primary_key)
       end
 
+      def belongs_to_or_through?
+        belongs_to? || (through_reflection && through_reflection.belongs_to_or_through?)
+      end
+
       # Returns the class name for the macro.
       #
       # <tt>composed_of :balance, class_name: 'Money'</tt> returns <tt>'Money'</tt>

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1166,4 +1166,33 @@ def test_build_for_has_many_through_association
     post_through = organization.posts.build
     assert_equal post_direct.author_id, post_through.author_id
   end
+
+  def test_single_has_many_through_association_with_unpersisted_parent_instance
+    post_with_single_has_many_through = Class.new(Post) do
+      def self.name; 'PostWithSingleHasManyThrough'; end
+      has_many :subscriptions, through: :author
+    end
+    post = post_with_single_has_many_through.new
+    post.author = Author.create!(name: 'Federico Morissette')
+    book = Book.create!(name: 'essays on single has many through associations')
+    post.author.books << book
+    subscription = Subscription.first
+    book.subscriptions << subscription
+    assert_equal [subscription], post.subscriptions.to_a
+  end
+
+  def test_nested_has_many_through_association_with_unpersisted_parent_instance
+    post_with_nested_has_many_through = Class.new(Post) do
+      def self.name; 'PostWithNestedHasManyThrough'; end
+      has_many :books, through: :author
+      has_many :subscriptions, through: :books
+    end
+    post = post_with_nested_has_many_through.new
+    post.author = Author.create!(name: 'Obie Weissnat')
+    book = Book.create!(name: 'essays on nested single has many through associations')
+    post.author.books << book
+    subscription = Subscription.first
+    book.subscriptions << subscription
+    assert_equal [subscription], post.subscriptions.to_a
+  end
 end