#cache_key delegates to #object by default

lib/active_model/serializer.rb

@@ -276,7 +276,7 @@ module ActiveModel
 
     def to_json(*args)
       if perform_caching?
-        cache.fetch expand_cache_key([self.class.to_s.underscore, object.cache_key, 'to-json']) do
+        cache.fetch expand_cache_key([self.class.to_s.underscore, cache_key, 'to-json']) do
           super
         end
       else
@@ -305,7 +305,7 @@ module ActiveModel
 <<<<<<< HEAD
 =======
       if perform_caching?
-        cache.fetch expand_cache_key([self.class.to_s.underscore, object.cache_key, 'serializable-hash']) do
+        cache.fetch expand_cache_key([self.class.to_s.underscore, cache_key, 'serializable-hash']) do
           _serializable_hash
         end
       else
@@ -441,7 +441,17 @@ module ActiveModel
     end
 
     def perform_caching?
-      perform_caching && cache && object.respond_to?(:cache_key)
+      perform_caching && cache && cache_key
+    end
+
+    # Override this method if you want to create a key based on associations
+    # Here's an example: your serializer has associations and the scope
+    #
+    # def cache_key
+    #   [ object, scope, scope.comments ]
+    # end
+    def cache_key
+      object.try :cache_key
     end
 
     def expand_cache_key(*args)

test/caching_test.rb

@@ -94,4 +94,27 @@ class CachingTest < ActiveModel::TestCase
       :skills => ['ruby'],
     }.to_json, serializer.cache.read('serializer/Adam/to-json'))
   end
+
+  def test_can_use_defined_cache_key
+    serializer = Class.new(ActiveModel::Serializer) do
+      cache true
+      attributes :name, :skills
+
+      def self.to_s
+        'serializer'
+      end
+
+      def cache_key
+        'custom-key'
+      end
+    end
+
+    serializer.cache = NullStore.new
+    instance = serializer.new Programmer.new
+
+    instance.to_json
+
+    assert serializer.cache.read('serializer/custom-key/to-json')
+    assert serializer.cache.read('serializer/custom-key/serializable-hash')
+  end
 end