Merge multi_fetch_fragments.

Makes caching a collection of template partials faster using `read_multi`
on the Rails cache store.

Some caching implementations have optimized `read_multi` so we don't have
to check in the cache store for every template.

actionview/lib/action_view/helpers/cache_helper.rb

@@ -161,6 +161,14 @@ module ActionView
         end
       end
 
+      # Given a key (as described in ActionController::Caching::Fragments.expire_fragment),
+      # returns a key suitable for use in reading, writing, or expiring a
+      # cached fragment. All keys are prefixed with <tt>views/</tt> and uses
+      # ActiveSupport::Cache.expand_cache_key for the expansion.
+      def fragment_cache_key(key)
+        ActiveSupport::Cache.expand_cache_key(key.is_a?(Hash) ? url_for(key).split("://").last : key, :views)
+      end
+
     private
 
       def fragment_name_with_digest(name) #:nodoc:

actionview/lib/action_view/railtie.rb

@@ -36,6 +36,12 @@ module ActionView
       end
     end
 
+    initializer "action_view.collection_caching" do |app|
+      ActiveSupport.on_load(:action_controller) do
+        PartialRenderer.collection_cache = app.config.action_controller.cache_store
+      end
+    end
+
     initializer "action_view.setup_action_pack" do |app|
       ActiveSupport.on_load(:action_controller) do
         ActionView::RoutingUrlFor.include(ActionDispatch::Routing::UrlFor)

actionview/lib/action_view/renderer/partial_renderer.rb

@@ -1,3 +1,4 @@
+require 'action_view/renderer/partial_renderer/collection_caching'
 require 'thread_safe'
 
 module ActionView
@@ -280,6 +281,8 @@ module ActionView
   #     <%- end -%>
   #   <% end %>
   class PartialRenderer < AbstractRenderer
+    include CollectionCaching
+
     PREFIXED_PARTIAL_NAMES = ThreadSafe::Cache.new do |h, k|
       h[k] = ThreadSafe::Cache.new
     end
@@ -321,8 +324,9 @@ module ActionView
         spacer = find_template(@options[:spacer_template], @locals.keys).render(@view, @locals)
       end
 
-      result = @template ? collection_with_template : collection_without_template
-      result.join(spacer).html_safe
+      cache_collection_render do
+        @template ? collection_with_template : collection_without_template
+      end.join(spacer).html_safe
     end
 
     def render_partial

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

@@ -0,0 +1,57 @@
+require 'active_support/core_ext/object/try'
+
+module ActionView
+  module CollectionCaching # :nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      # Fallback cache store if Action View is used without Rails.
+      # Otherwise overriden in Railtie to use Rails.cache.
+      mattr_accessor(:collection_cache) { ActiveSupport::Cache::MemoryStore.new }
+    end
+
+    private
+      def cache_collection_render
+        return yield unless cache_collection?
+
+        keyed_collection = collection_by_cache_keys
+        partial_cache = collection_cache.read_multi(*keyed_collection.keys)
+
+        @collection = keyed_collection.reject { |key, _| partial_cache.key?(key) }.values
+        rendered_partials = @collection.any? ? yield.dup : []
+
+        fetch_or_cache_partial(partial_cache, order_by: keyed_collection.each_key) do
+          rendered_partials.shift
+        end
+      end
+
+      def cache_collection?
+        @options[:cache].present?
+      end
+
+      def collection_by_cache_keys
+        seed = @options[:cache].respond_to?(:call) ? @options[:cache] : ->(i) { i }
+
+        @collection.each_with_object({}) do |item, hash|
+          hash[expanded_cache_key(seed.call(item))] = item
+        end
+      end
+
+      def expanded_cache_key(key)
+        key = @view.fragment_cache_key(@view.cache_fragment_name(key))
+        key.frozen? ? key.dup : key # #read_multi & #write may require mutability, Dalli 2.6.0.
+      end
+
+      def fetch_or_cache_partial(cached_partials, order_by:)
+        cache_options = @options[:cache_options] || @locals[:cache_options] || {}
+
+        order_by.map do |key|
+          cached_partials.fetch(key) do
+            yield.tap do |rendered_partial|
+              collection_cache.write(key, rendered_partial, cache_options)
+            end
+          end
+        end
+      end
+  end
+end

actionview/test/actionpack/controller/render_test.rb

@@ -31,6 +31,10 @@ class Customer < Struct.new(:name, :id)
   def persisted?
     id.present?
   end
+
+  def cache_key
+    name.to_s
+  end
 end
 
 module Quiz

actionview/test/template/render_test.rb

@@ -597,3 +597,17 @@ class LazyViewRenderTest < ActiveSupport::TestCase
     silence_warnings { Encoding.default_external = old }
   end
 end
+
+class CachedCollectionViewRenderTest < CachedViewRenderTest
+  test "with custom key" do
+    customer = Customer.new("david")
+    key = ActionController::Base.new.fragment_cache_key([customer, 'key'])
+
+    ActionView::PartialRenderer.collection_cache.write(key, 'Hello')
+
+    assert_equal "Hello",
+      @view.render(partial: "test/customer", collection: [customer], cache: ->(item) { [item, 'key'] })
+
+    ActionView::PartialRenderer.collection_cache.clear
+  end
+end