Only write to collection cache if we have a callable cache key.

A callable cache key writes to the collection cache under a certain namespace.
Which means if we don't have scoped cache key we can just rely on the
`cache model_name do` in the templates to cache them.

Less writes, more sharing.

Add `assert_customer_cached` to better illustrate this in tests, and remove
tests which then don't communicate as much.

actionpack/test/controller/caching_test.rb

@@ -410,6 +410,7 @@ class AutomaticCollectionCacheTest < ActionController::TestCase
   def test_collection_fetches_cached_views
     get :index
     assert_equal 1, @controller.partial_rendered_times
+    assert_customer_cached 'david/1', 'david, 1'
 
     get :index
     assert_equal 1, @controller.partial_rendered_times
@@ -441,23 +442,15 @@ class AutomaticCollectionCacheTest < ActionController::TestCase
 
   def test_caching_with_callable_cache_key
     get :index_with_callable_cache_key
-    assert_equal 1, @controller.partial_rendered_times
-    assert_select ':root', 'david, 1'
-
-    get :index_with_callable_cache_key
-    assert_equal 1, @controller.partial_rendered_times
-    assert_select ':root', 'david, 1'
+    assert_customer_cached 'cached_david', 'david, 1'
+    assert_customer_cached 'david/1', 'david, 1'
   end
 
-  def test_caching_mixing_callable_cache_key_and_automatic_caching
-    get :index
-    assert_equal 1, @controller.partial_rendered_times
-    assert_select ':root', 'david, 1'
-
-    get :index_with_callable_cache_key
-    assert_equal 1, @controller.partial_rendered_times, 'individual cache not reused with collection'
-    assert_select ':root', 'david, 1'
-  end
+  private
+    def assert_customer_cached(key, content)
+      assert_match content,
+        ActionView::PartialRenderer.collection_cache.read("views/#{key}/7c228ab609f0baf0b1f2367469210937")
+    end
 end
 
 class FragmentCacheKeyTestController < CachingController

actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb

@@ -51,10 +51,14 @@ module ActionView
       end
 
       def fetch_or_cache_partial(cached_partials, order_by:)
+        rely_on_individual_cache_call = !callable_cache_key?
+
         order_by.map do |key|
           cached_partials.fetch(key) do
             yield.tap do |rendered_partial|
-              collection_cache.write(key, rendered_partial, @options[:cache_options])
+              unless rely_on_individual_cache_call
+                collection_cache.write(key, rendered_partial, @options[:cache_options])
+              end
             end
           end
         end