[Fix rails#33155] Set through target for new records

Author: joshuay03

activerecord/lib/active_record/associations/association.rb

@@ -188,6 +188,9 @@ def extensions
       # not reraised. The proxy is \reset and +nil+ is the return value.
       def load_target
         @target = find_target(async: false) if (@stale_state && stale_target?) || find_target?
+        if !@target && set_through_target_for_new_record?
+          @target = through_association.target.association(reflection.source_reflection_name).target
+        end
 
         loaded! unless loaded?
         target
@@ -321,6 +324,10 @@ def find_target?
           !loaded? && (!owner.new_record? || foreign_key_present?) && klass
         end
 
+        def set_through_target_for_new_record?
+          owner.new_record? && reflection.through_reflection? && through_association.target
+        end
+
         # Returns true if there is a foreign key present on the owner which
         # references the target. This is used to determine whether we can load
         # the target if the owner is currently a new record (and therefore

activerecord/lib/active_record/associations/collection_association.rb

@@ -272,6 +272,14 @@ def include?(record)
       def load_target
         if find_target?
           @target = merge_target_lists(find_target, target)
+        elsif target.empty? && set_through_target_for_new_record?
+          @target = if through_reflection.collection?
+            through_association.target.flat_map do |record|
+              record.association(reflection.source_reflection_name).target
+            end
+          else
+            through_association.target.association(reflection.source_reflection_name).target
+          end
         end
 
         loaded!

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -56,6 +56,20 @@ def setup
     Reader.create person_id: 0, post_id: 0
   end
 
+  def test_setting_association_on_new_record_sets_through_records
+    subscriber_1, subscriber_2 = Subscriber.create!(nick: "nick 1"), Subscriber.create!(nick: "nick 2")
+    subscription_1 = Subscription.new(subscriber: subscriber_1)
+    subscription_2 = Subscription.new(subscriber: subscriber_2)
+    book = Book.new
+    book.subscriptions = [subscription_1, subscription_2]
+
+    assert_predicate subscriber_1, :persisted?
+    assert_predicate subscriber_2, :persisted?
+    assert_predicate book, :new_record?
+    book.subscriptions.each { |subscription| assert_predicate subscription, :new_record? }
+    assert_equal book.subscribers.sort, [subscriber_1, subscriber_2].sort
+  end
+
   def test_has_many_through_create_record
     assert books(:awdr).subscribers.create!(nick: "bob")
   end

activerecord/test/cases/associations/has_one_through_associations_test.rb

@@ -43,6 +43,18 @@ def test_has_one_through_executes_limited_query
     end
   end
 
+  def test_setting_association_on_new_record_sets_through_record
+    club = Club.create!
+    membership = CurrentMembership.new(club: club)
+    member = Member.new
+    member.current_membership = membership
+
+    assert_predicate club, :persisted?
+    assert_predicate member, :new_record?
+    assert_predicate member.current_membership, :new_record?
+    assert_equal club, member.club
+  end
+
   def test_creating_association_creates_through_record
     new_member = Member.create(name: "Chris")
     new_member.club = Club.create(name: "LRUG")