Handle polymorphic collection associations when setting through records

Author: joshuay03

activerecord/lib/active_record/associations/collection_association.rb

@@ -279,7 +279,7 @@ def load_target
             if middle_target.nil? || (middle_reflection.collection? && middle_target.empty?)
               break []
             elsif middle_reflection.collection?
-              middle_target.flat_map { |record| record.association(through_reflection.source_reflection_name).load_target }
+              middle_target.flat_map { |record| record.association(through_reflection.source_reflection_name).load_target }.compact
             else
               middle_target.association(through_reflection.source_reflection_name).load_target
             end

activerecord/lib/active_record/reflection.rb

@@ -1235,7 +1235,7 @@ def derive_class_name
 
     class PolymorphicReflection < AbstractReflection # :nodoc:
       delegate :klass, :scope, :plural_name, :type, :join_primary_key, :join_foreign_key,
-               :name, :scope_for, to: :@reflection
+               :name, :scope_for, :collection?, to: :@reflection
 
       def initialize(reflection, previous_reflection)
         super()

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -84,6 +84,44 @@ def test_setting_has_many_through_many_association_on_new_record_sets_through_re
     assert_no_queries { assert_equal [subscriber_1, subscriber_2].sort, book.subscribers.sort }
   end
 
+  def test_setting_has_many_through_many_association_with_missing_targets_on_new_record_sets_empty_through_records
+    subscription_1 = Subscription.new
+    subscription_2 = Subscription.new
+    book = Book.new
+    book.subscriptions = [subscription_1, subscription_2]
+
+    assert_predicate book, :new_record?
+    book.subscriptions.each { |subscription| assert_predicate subscription, :new_record? }
+    assert_no_queries { assert_equal [], book.subscribers }
+  end
+
+  def test_setting_has_many_through_many_association_with_partial_missing_targets_on_new_record_sets_partial_through_records
+    subscriber_1 = Subscriber.create!(nick: "nick 1")
+    subscription_1 = Subscription.new(subscriber: subscriber_1)
+    subscription_2 = Subscription.new
+    book = Book.new
+    book.subscriptions = [subscription_1, subscription_2]
+
+    assert_predicate subscriber_1, :persisted?
+    assert_predicate book, :new_record?
+    book.subscriptions.each { |subscription| assert_predicate subscription, :new_record? }
+    assert_no_queries { assert_equal [subscriber_1], book.subscribers }
+  end
+
+  def test_setting_polymorphic_has_many_through_many_association_on_new_record_sets_through_records
+    human_1, human_2 = Human.create!, Human.create!
+    interest_1 = Interest.new(polymorphic_human: human_1)
+    interest_2 = Interest.new(polymorphic_human: human_2)
+    zine = Zine.new
+    zine.interests = [interest_1, interest_2]
+
+    assert_predicate human_1, :persisted?
+    assert_predicate human_2, :persisted?
+    assert_predicate zine, :new_record?
+    zine.interests.each { |interest| assert_predicate interest, :new_record? }
+    assert_no_queries { assert_equal [human_1, human_2].sort, zine.polymorphic_humans.sort }
+  end
+
   def test_setting_nested_has_many_through_one_association_on_new_record_sets_nested_through_records
     post_tagging_1, post_tagging_2 = Tagging.create!, Tagging.create!
     post = Post.create!(title: "Tagged", body: "Post", taggings: [post_tagging_1, post_tagging_2])