Bugfix foreign key replacement in inverse association

when model is added to collection association

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -12,17 +12,20 @@ module ActiveRecord
         if record
           raise_on_type_mismatch!(record)
           update_counters_on_replace(record)
-          replace_keys(record)
           set_inverse_instance(record)
           @updated = true
         else
           decrement_counters
-          remove_keys
         end
 
         self.target = record
       end
 
+      def target=(record)
+        replace_keys(record)
+        super
+      end
+
       def default(&block)
         writer(owner.instance_exec(&block)) if reader.nil?
       end
@@ -78,11 +81,8 @@ module ActiveRecord
         end
 
         def replace_keys(record)
-          owner[reflection.foreign_key] = record._read_attribute(reflection.association_primary_key(record.class))
+          owner[reflection.foreign_key] = record ?
-        end
+            record._read_attribute(reflection.association_primary_key(record.class)) : nil
-
-        def remove_keys
-          owner[reflection.foreign_key] = nil
         end
 
         def foreign_key_present?

activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb

@@ -13,12 +13,7 @@ module ActiveRecord
 
         def replace_keys(record)
           super
-          owner[reflection.foreign_type] = record.class.base_class.name
+          owner[reflection.foreign_type] = record ? record.class.base_class.name : nil
-        end
-
-        def remove_keys
-          super
-          owner[reflection.foreign_type] = nil
         end
 
         def different_target?(record)

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2509,6 +2509,15 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_same car, new_bulb.car
   end
 
+  test "reattach to new objects replaces inverse association and foreign key" do
+    bulb = Bulb.create!(car: Car.create!)
+    assert bulb.car_id
+    car = Car.new
+    car.bulbs << bulb
+    assert_equal car, bulb.car
+    assert_nil bulb.car_id
+  end
+
   test "in memory replacement maintains order" do
     first_bulb = Bulb.create!
     second_bulb = Bulb.create!