Raise an exception with friendlier error message when attempting to build a polymorphic belongs_to with accepts_nested_attributes_for. [#2318 state:resolved]

Signed-off-by: Eloy Duran <eloy.de.enige@gmail.com>

activerecord/lib/active_record/nested_attributes.rb

@@ -260,7 +260,12 @@ def assign_nested_attributes_for_one_to_one_association(association_name, attrib
 
       if attributes['id'].blank?
         unless reject_new_record?(association_name, attributes)
-          send("build_#{association_name}", attributes.except(*UNASSIGNABLE_KEYS))
+          method = "build_#{association_name}"
+          if respond_to?(method)
+            send(method, attributes.except(*UNASSIGNABLE_KEYS))
+          else
+            raise ArgumentError, "Cannot build association #{association_name}. Are you trying to build a polymorphic one-to-one association?"
+          end
         end
       elsif (existing_record = send(association_name)) && existing_record.id.to_s == attributes['id'].to_s
         assign_to_or_mark_for_destruction(existing_record, attributes, allow_destroy)

activerecord/test/cases/nested_attributes_test.rb

@@ -81,11 +81,19 @@ def test_a_model_should_respond_to_underscore_delete_and_return_if_it_is_marked_
 end
 
 class TestNestedAttributesOnAHasOneAssociation < ActiveRecord::TestCase
+  include AssertRaiseWithMessage
+
   def setup
     @pirate = Pirate.create!(:catchphrase => "Don' botharrr talkin' like one, savvy?")
     @ship = @pirate.create_ship(:name => 'Nights Dirty Lightning')
   end
 
+  def test_should_raise_argument_error_if_trying_to_build_polymorphic_belongs_to
+    assert_raise_with_message ArgumentError, "Cannot build association looter. Are you trying to build a polymorphic one-to-one association?" do
+      Treasure.new(:name => 'pearl', :looter_attributes => {:catchphrase => "Arrr"})
+    end
+  end
+
   def test_should_define_an_attribute_writer_method_for_the_association
     assert_respond_to @pirate, :ship_attributes=
   end

activerecord/test/models/treasure.rb

@@ -3,4 +3,6 @@ class Treasure < ActiveRecord::Base
   belongs_to :looter, :polymorphic => true
 
   has_many :price_estimates, :as => :estimate_of
+
+  accepts_nested_attributes_for :looter
 end