Fix nested `has many :through` associations on unpersisted instances

Fixes: #16313

3 files changed, 34 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/through_association.rb b/activerecord/lib/active_record/associations/through_association.rb
index af1bce523c..a276907dd3 100644
--- a/activerecord/lib/active_record/associations/through_association.rb
+++ b/activerecord/lib/active_record/associations/through_association.rb
@@ -77,7 +77,7 @@ module ActiveRecord
         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
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 4265afc0a5..09d61b47a2 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -139,6 +139,10 @@ module ActiveRecord
         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>
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index 5f52c65412..83365884f5 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -1166,4 +1166,33 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     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