Do not overwrite manually built records during one-to-one nested attribute assignment

For one-to-one nested associations, if you build the new (in-memory)
child object yourself before assignment, then the NestedAttributes
module will not overwrite it, e.g.:

    class Member < ActiveRecord::Base
      has_one :avatar
      accepts_nested_attributes_for :avatar

      def avatar
        super || build_avatar(width: 200)
      end
    end

    member = Member.new
    member.avatar_attributes = {icon: 'sad'}
    member.avatar.width # => 200

1 files changed, 14 insertions, 0 deletions

diff --git a/activerecord/test/cases/nested_attributes_test.rb b/activerecord/test/cases/nested_attributes_test.rb
index b6e140b912..165b7454b7 100644
--- a/activerecord/test/cases/nested_attributes_test.rb
+++ b/activerecord/test/cases/nested_attributes_test.rb
@@ -131,6 +131,20 @@ class TestNestedAttributesInGeneral < ActiveRecord::TestCase
     assert_equal 's1', ship.reload.name
   end
 
+  def test_reuse_already_built_new_record
+    pirate = Pirate.new
+    ship_built_first = pirate.build_ship
+    pirate.ship_attributes = { name: 'Ship 1' }
+    assert_equal ship_built_first.object_id, pirate.ship.object_id
+  end
+
+  def test_do_not_allow_assigning_foreign_key_when_reusing_existing_new_record
+    pirate = Pirate.create!(catchphrase: "Don' botharrr talkin' like one, savvy?")
+    pirate.build_ship
+    pirate.ship_attributes = { name: 'Ship 1', pirate_id: pirate.id + 1 }
+    assert_equal pirate.id, pirate.ship.pirate_id
+  end
+
   def test_reject_if_with_a_proc_which_returns_true_always_for_has_many
     Man.accepts_nested_attributes_for :interests, :reject_if => proc {|attributes| true }
     man = Man.create(name: "John")