Cached collections only work if there is one template

Cached collections only work if there is one template.  If there are
more than one templates, the caching mechanism doesn't have a key.

actionview/lib/action_view/renderer/partial_renderer.rb

@@ -308,6 +308,9 @@ def render(context, options, block)
         template = find_partial(@path, @template_keys)
         @variable ||= template.variable
       else
+        if options[:cached]
+          raise NotImplementedError, "render caching requires a template. Please specify a partial when rendering"
+        end
         template = nil
       end
 
@@ -337,9 +340,14 @@ def render_collection(view, template)
             spacer = find_template(@options[:spacer_template], @locals.keys).render(view, @locals)
           end
 
-          cache_collection_render(payload, view, template) do
-            template ? collection_with_template(view, template) : collection_without_template(view)
-          end.join(spacer).html_safe
+          collection_body = if template
+            cache_collection_render(payload, view, template) do
+              collection_with_template(view, template)
+            end
+          else
+            collection_without_template(view)
+          end
+          collection_body.join(spacer).html_safe
         end
       end
 

actionview/test/template/render_test.rb

@@ -752,6 +752,17 @@ class CachedCustomer < Customer; end
       @view.render(partial: "test/cached_customer", collection: [customer], cached: true)
   end
 
+  test "collection caching does not work on multi-partials" do
+    a = Object.new
+    b = Object.new
+    def a.to_partial_path; "test/partial_iteration_1"; end
+    def b.to_partial_path; "test/partial_iteration_2"; end
+
+    assert_raises(NotImplementedError) do
+      @controller_view.render(partial: [a, b], cached: true)
+    end
+  end
+
   private
     def cache_key(*names, virtual_path)
       digest = ActionView::Digestor.digest name: virtual_path, finder: @view.lookup_context, dependencies: []