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 %>
```

7 files changed, 101 insertions, 3 deletions

diff --git a/actionview/lib/action_view/helpers/cache_helper.rb b/actionview/lib/action_view/helpers/cache_helper.rb
index 56b1c5049c..dec8c9cee9 100644
--- a/actionview/lib/action_view/helpers/cache_helper.rb
+++ b/actionview/lib/action_view/helpers/cache_helper.rb
@@ -110,6 +110,29 @@ module ActionView
       #   <%= some_helper_method(person) %>
       #
       # Now all you'll have to do is change that timestamp when the helper method changes.
+      #
+      # === Automatic Collection Caching
+      #
+      # When rendering collections such as:
+      #
+      #   <%= render @notifications %>
+      #   <%= render partial: 'notifications/notification', collection: @notifications %>
+      #
+      # If the notifications/_notification partial starts with a cache call like so:
+      #
+      #   <% cache notification do %>
+      #     <%= notification.name %>
+      #   <% end %>
+      #
+      # The collection can then automatically use any cached renders for that
+      # template by reading them at once instead of one by one.
+      #
+      # See ActionView::Template::Handlers::ERB.resource_cache_call_pattern for more
+      # information on what cache calls make a template eligible for this collection caching.
+      #
+      # The automatic cache multi read can be turned off like so:
+      #
+      #   <%= render @notifications, cache: false %>
       def cache(name = {}, options = nil, &block)
         if controller.perform_caching
           safe_concat(fragment_for(cache_fragment_name(name, options), options, &block))
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
diff --git a/actionview/lib/action_view/template.rb b/actionview/lib/action_view/template.rb
index 6b61378a1f..4eae3e2830 100644
--- a/actionview/lib/action_view/template.rb
+++ b/actionview/lib/action_view/template.rb
@@ -117,6 +117,7 @@ module ActionView
       @source            = source
       @identifier        = identifier
       @handler           = handler
+      @cache_name        = extract_resource_cache_call_name
       @compiled          = false
       @original_encoding = nil
       @locals            = details[:locals] || []
@@ -152,6 +153,10 @@ module ActionView
       @type ||= Types[@formats.first] if @formats.first
     end
 
+    def eligible_for_collection_caching?(as: nil)
+      @cache_name == (as || inferred_cache_name).to_s
+    end
+
     # Receives a view object and return a template similar to self by using @virtual_path.
     #
     # This method is useful if you have a template object but it does not contain its source
@@ -332,5 +337,14 @@ module ActionView
         payload = { virtual_path: @virtual_path, identifier: @identifier }
         ActiveSupport::Notifications.instrument("#{action}.action_view", payload, &block)
       end
+
+      def extract_resource_cache_call_name
+        $1 if @handler.respond_to?(:resource_cache_call_pattern) &&
+          @source =~ @handler.resource_cache_call_pattern
+      end
+
+      def inferred_cache_name
+        @inferred_cache_name ||= @virtual_path.split('/').last.sub('_', '')
+      end
   end
 end
diff --git a/actionview/lib/action_view/template/handlers/erb.rb b/actionview/lib/action_view/template/handlers/erb.rb
index 85a100ed4c..88a8570706 100644
--- a/actionview/lib/action_view/template/handlers/erb.rb
+++ b/actionview/lib/action_view/template/handlers/erb.rb
@@ -123,6 +123,24 @@ module ActionView
           ).src
         end
 
+        # Returns Regexp to extract a cached resource's name from a cache call at the
+        # first line of a template.
+        # The extracted cache name is expected in $1.
+        #
+        #   <% cache notification do %> # => notification
+        #
+        # The pattern should support templates with a beginning comment:
+        #
+        #   <%# Still extractable even though there's a comment %>
+        #   <% cache notification do %> # => notification
+        #
+        # But fail to extract a name if a resource association is cached.
+        #
+        #   <% cache notification.event do %> # => nil
+        def resource_cache_call_pattern
+          /\A(?:<%#.*%>\n?)?<% cache\(?\s*(\w+\.?)/
+        end
+
       private
 
         def valid_encoding(string, encoding)
diff --git a/actionview/test/fixtures/test/_cached_customer.erb b/actionview/test/fixtures/test/_cached_customer.erb
new file mode 100644
index 0000000000..52f35a3497
--- /dev/null
+++ b/actionview/test/fixtures/test/_cached_customer.erb
@@ -0,0 +1,3 @@
+<% cache cached_customer do %>
+  Hello: <%= cached_customer.name %>
+<% end %>
\ No newline at end of file
diff --git a/actionview/test/fixtures/test/_cached_customer_as.erb b/actionview/test/fixtures/test/_cached_customer_as.erb
new file mode 100644
index 0000000000..fca8d19e34
--- /dev/null
+++ b/actionview/test/fixtures/test/_cached_customer_as.erb
@@ -0,0 +1,3 @@
+<% cache buyer do %>
+  <%= greeting %>: <%= customer.name %>
+<% end %>
\ No newline at end of file
diff --git a/actionview/test/template/render_test.rb b/actionview/test/template/render_test.rb
index 73871cf974..691636a8f9 100644
--- a/actionview/test/template/render_test.rb
+++ b/actionview/test/template/render_test.rb
@@ -599,6 +599,12 @@ class LazyViewRenderTest < ActiveSupport::TestCase
 end
 
 class CachedCollectionViewRenderTest < CachedViewRenderTest
+  class CachedCustomer < Customer; end
+
+  teardown do
+    ActionView::PartialRenderer.collection_cache.clear
+  end
+
   test "with custom key" do
     customer = Customer.new("david")
     key = ActionController::Base.new.fragment_cache_key([customer, 'key'])
@@ -607,7 +613,25 @@ class CachedCollectionViewRenderTest < CachedViewRenderTest
 
     assert_equal "Hello",
       @view.render(partial: "test/customer", collection: [customer], cache: ->(item) { [item, 'key'] })
+  end
 
-    ActionView::PartialRenderer.collection_cache.clear
+  test "automatic caching with inferred cache name" do
+    customer = CachedCustomer.new("david")
+    key = ActionController::Base.new.fragment_cache_key(customer)
+
+    ActionView::PartialRenderer.collection_cache.write(key, 'Cached')
+
+    assert_equal "Cached",
+      @view.render(partial: "test/cached_customer", collection: [customer])
+  end
+
+  test "automatic caching with as name" do
+    customer = CachedCustomer.new("david")
+    key = ActionController::Base.new.fragment_cache_key(customer)
+
+    ActionView::PartialRenderer.collection_cache.write(key, 'Cached')
+
+    assert_equal "Cached",
+      @view.render(partial: "test/cached_customer_as", collection: [customer], as: :buyer)
   end
 end