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.

3 files changed, 29 insertions, 3 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index fc726d6519..efc10913f4 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   `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*
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index b3c3e26c9f..dd92e29199 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -184,9 +184,7 @@ module ActiveRecord
             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 @@ module ActiveRecord
 
               raise ActiveRecord::Rollback unless saved
             end
+            @new_record_before_save = false
           end
 
           # reconstruct the scope now that we know the owner's id
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