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.

activerecord/CHANGELOG.md

+*   `has_many :through` associations will no longer save the through record
+    twice when added in an `after_create` callback defined before the
+    associations.
+
+    Fixes #3798.
+
+    *Sean Griffin*
+
 *   Add `bin/rake db:purge` task to empty the current database.
 
     *Yves Senn*

activerecord/lib/active_record/autosave_association.rb

@@ -184,9 +184,7 @@ def add_autosave_association_callbacks(reflection)
             before_save :before_save_collection_association
 
             define_non_cyclic_method(save_method) { save_collection_association(reflection) }
-            # Doesn't use after_save as that would save associations added in after_create/after_update twice
-            after_create save_method
-            after_update save_method
+            after_save save_method
           elsif reflection.has_one?
             define_method(save_method) { save_has_one_association(reflection) } unless method_defined?(save_method)
             # Configures two callbacks instead of a single after_save so that
@@ -364,6 +362,7 @@ def save_collection_association(reflection)
 
               raise ActiveRecord::Rollback unless saved
             end
+            @new_record_before_save = false
           end
 
           # reconstruct the scope now that we know the owner's id

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1139,4 +1139,23 @@ def test_has_many_through_unscope_default_scope
     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