Fix callbacks on has_many :through associations (#33249)

When adding a child record via a has_many :through association,
build_through_record would previously build the join record, and then
assign the child record and source_type option to it.  Because the
before_add and after_add callbacks are called as part of build, however,
this caused the callbacks to receive incomplete records, specifically
without the other end of the has_many :through association.  Collecting
all attributes before building the join record ensures the callbacks
receive the fully constructed record.

1 files changed, 18 insertions, 0 deletions

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 67e013c6e0..fa540c9dab 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -1474,6 +1474,24 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal [subscription2], post.subscriptions.to_a
   end
 
+  def test_child_is_visible_to_join_model_in_add_association_callbacks
+    [:before_add, :after_add].each do |callback_name|
+      sentient_treasure = Class.new(Treasure) do
+        def self.name; "SentientTreasure"; end
+
+        has_many :pet_treasures, foreign_key: :treasure_id, callback_name => :check_pet!
+        has_many :pets, through: :pet_treasures
+
+        def check_pet!(added)
+          raise "No pet!" if added.pet.nil?
+        end
+      end
+
+      treasure = sentient_treasure.new
+      assert_nothing_raised { treasure.pets << pets(:mochi) }
+    end
+  end
+
   private
     def make_model(name)
       Class.new(ActiveRecord::Base) { define_singleton_method(:name) { name } }