Allow polymorphic routes with nil when a route can still be drawn

Suppose you have two resources routed in the following manner:

```ruby
resources :blogs do
  resources :posts
end

resources :posts
```

When using polymorphic resource routing like `url_for([@blog, @post])`, and `@blog` is `nil` Rails should still try to match the route to the top-level posts resource.

Fixes #16754

1 files changed, 34 insertions, 2 deletions

diff --git a/actionview/test/activerecord/polymorphic_routes_test.rb b/actionview/test/activerecord/polymorphic_routes_test.rb
index 0d97ddb2f6..a259c359f0 100644
--- a/actionview/test/activerecord/polymorphic_routes_test.rb
+++ b/actionview/test/activerecord/polymorphic_routes_test.rb
@@ -183,16 +183,33 @@ class PolymorphicRoutesTest < ActionController::TestCase
     end
   end
 
-  def test_with_nil_in_list
+  def test_with_entirely_nil_list
     with_test_routes do
       exception = assert_raise ArgumentError do
         @series.save
-        polymorphic_url([nil, @series])
+        polymorphic_url([nil, nil])
       end
       assert_equal "Nil location provided. Can't build URI.", exception.message
     end
   end
 
+  def test_with_nil_in_list_for_resource_that_could_be_top_level_or_nested
+    with_top_level_and_nested_routes do
+      @blog_post.save
+      assert_equal "http://example.com/posts/#{@blog_post.id}", polymorphic_url([nil, @blog_post])
+    end
+  end
+  
+  def test_with_nil_in_list_does_not_generate_invalid_link
+    with_top_level_and_nested_routes do
+      exception = assert_raise NoMethodError do
+        @series.save
+        polymorphic_url([nil, @series])
+      end
+      assert_match /undefined method `series_url' for/, exception.message
+    end
+  end
+
   def test_with_record
     with_test_routes do
       @project.save
@@ -626,6 +643,21 @@ class PolymorphicRoutesTest < ActionController::TestCase
     end
   end
 
+  def with_top_level_and_nested_routes(options = {})
+    with_routing do |set|
+      set.draw do
+        resources :blogs do
+          resources :posts
+          resources :series
+        end
+        resources :posts
+      end
+
+      extend @routes.url_helpers
+      yield
+    end
+  end
+
   def with_admin_test_routes(options = {})
     with_routing do |set|
       set.draw do