When caching, return the object's cache_key up front if it's defined.

This will prevent objects PORO objects that don't have updated_at defined, from throwing an error.

Not as big a deal now that PORO objects can inherit ActiveModelSerializers::Model, but still necessary if it's not inherited for whatever reason.

Add the Adapter type to the cache key.

This prevents incorrect results when the same object is serialized with different adapters.

BF:

Cherry-pick of
040a97b9e9
which was a squash of
f89ed71058

from pr 1346

lib/active_model/serializer/caching.rb

@@ -7,7 +7,7 @@ module ActiveModel
         with_options instance_writer: false, instance_reader: false do |serializer|
           serializer.class_attribute :_cache         # @api private : the cache store
           serializer.class_attribute :_fragmented    # @api private : @see ::fragmented
-          serializer.class_attribute :_cache_key     # @api private : when present, is first item in cache_key
+          serializer.class_attribute :_cache_key     # @api private : when present, is first item in cache_key.  Ignored if the serializable object defines #cache_key.
           serializer.class_attribute :_cache_only    # @api private : when fragment caching, whitelists cached_attributes. Cannot combine with except
           serializer.class_attribute :_cache_except  # @api private : when fragment caching, blacklists cached_attributes. Cannot combine with only
           serializer.class_attribute :_cache_options # @api private : used by CachedSerializer, passed to _cache.fetch
@@ -58,6 +58,10 @@ module ActiveModel
           ''.freeze
         end
 
+        def _skip_digest?
+          _cache_options && _cache_options[:skip_digest]
+        end
+
         # @api private
         # Used by FragmentCache on the CachedSerializer
         #  to call attribute methods on the fragmented cached serializer.
@@ -142,6 +146,18 @@ module ActiveModel
             (_cache_only && !_cache_except || !_cache_only && _cache_except)
         end
       end
+
+      # Use object's cache_key if available, else derive a key from the object
+      # Pass the `key` option to the `cache` declaration or override this method to customize the cache key
+      def cache_key
+        if object.respond_to?(:cache_key)
+          object.cache_key
+        else
+          object_time_safe = object.updated_at
+          object_time_safe = object_time_safe.strftime('%Y%m%d%H%M%S%9N') if object_time_safe.respond_to?(:strftime)
+          "#{self.class._cache_key}/#{object.id}-#{object_time_safe}"
+        end
+      end
     end
   end
 end