Don't save through records twice

If the through record gets created in an `after_create` hook that is
defined before the association is defined (therefore after its
`after_create` hook) get saved twice. This ensures that the through
records are created only once, regardless of the order of the hooks.

1 files changed, 19 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 8641584c0c..6895df73c1 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -1139,4 +1139,23 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal 2, post.lazy_readers_unscope_skimmers.to_a.size
     assert_equal 2, post.lazy_people_unscope_skimmers.to_a.size
   end
+
+  class ClubWithCallbacks < ActiveRecord::Base
+    self.table_name = 'clubs'
+    after_create :add_a_member
+
+    has_many :memberships, inverse_of: :club, foreign_key: :club_id
+    has_many :members, through: :memberships
+
+    def add_a_member
+      members << Member.last
+    end
+  end
+
+  def test_has_many_with_callback_before_association
+    Member.create!
+    club = ClubWithCallbacks.create!
+
+    assert_equal 1, club.reload.memberships.count
+  end
 end