Collections automatically cache and fetch partials.

Collections can take advantage of `multi_read` if they render one template
and their partials begin with a cache call.

The cache call must correspond to either what the collections elements are
rendered as, or match the inferred name of the partial.

So with a notifications/_notification.html.erb template like:

```ruby
<% cache notification %>
  <%# ... %>
<% end %>
```

A collection would be able to use `multi_read` if rendered like:

```ruby
<%= render @notifications %>
<%= render partial: 'notifications/notification', collection: @notifications, as: :notification %>
```

1 files changed, 15 insertions, 2 deletions

diff --git a/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb b/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
index 0f2ab887bc..b77c884e66 100644
--- a/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
+++ b/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
@@ -26,11 +26,24 @@ module ActionView
       end
 
       def cache_collection?
-        @options[:cache].present?
+        @options.fetch(:cache, automatic_cache_eligible?)
+      end
+
+      def automatic_cache_eligible?
+        single_template_render? && !callable_cache_key? &&
+          @template.eligible_for_collection_caching?(as: @options[:as])
+      end
+
+      def single_template_render?
+        @template # Template is only set when a collection renders one template.
+      end
+
+      def callable_cache_key?
+        @options[:cache].respond_to?(:call)
       end
 
       def collection_by_cache_keys
-        seed = @options[:cache].respond_to?(:call) ? @options[:cache] : ->(i) { i }
+        seed = callable_cache_key? ? @options[:cache] : ->(i) { i }
 
         @collection.each_with_object({}) do |item, hash|
           hash[expanded_cache_key(seed.call(item))] = item