When assigning a has_one, if the existing record fails to be removed …

…from the association, raise an error

activerecord/lib/active_record/associations/has_one_association.rb

@@ -20,7 +20,7 @@ def replace(record, save = true)
         load_target
 
         if @target && @target != record
-          remove_target(@reflection.options[:dependent])
+          remove_target!(@reflection.options[:dependent])
         end
 
         if record
@@ -55,13 +55,19 @@ def new_record(method, attributes)
           record
         end
 
-        def remove_target(method)
-          case method
-          when :delete, :destroy
+        def remove_target!(method)
+          if [:delete, :destroy].include?(method)
             @target.send(method)
           else
             @target[@reflection.foreign_key] = nil
-            @target.save if @target.persisted? && @owner.persisted?
+
+            if @target.persisted? && @owner.persisted?
+              unless @target.save
+                @target[@reflection.foreign_key] = @target.send("#{@reflection.foreign_key}_was")
+                raise RecordNotSaved, "Failed to remove the existing associated #{@reflection.name}. " +
+                                      "The record failed to save when after its foreign key was set to nil."
+              end
+            end
           end
         end
     end

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -308,4 +308,16 @@ def test_creation_failure_with_dependent_option
     assert new_ship.new_record?
     assert orig_ship.destroyed?
   end
+
+  def test_replacement_failure_due_to_existing_record_should_raise_error
+    pirate = pirates(:blackbeard)
+    pirate.ship.name = nil
+
+    assert !pirate.ship.valid?
+    assert_raise(ActiveRecord::RecordNotSaved) do
+      pirate.ship = ships(:interceptor)
+    end
+    assert_equal ships(:black_pearl), pirate.ship
+    assert_equal pirate.id, pirate.ship.pirate_id
+  end
 end