Use ActiveSupport::Cache.expand_cache_key for cache key expansions (#1878)

* Use ActiveSupport::Cache.expand_cache_key for cache key expansions

test/cache_test.rb

@@ -4,6 +4,30 @@ require 'tempfile'
 
 module ActiveModelSerializers
   class CacheTest < ActiveSupport::TestCase
+    # Instead of a primitive cache key (i.e. a string), this class
+    # returns a list of objects that require to be expanded themselves.
+    class AuthorWithExpandableCacheElements < Author
+      # For the test purposes it's important that #to_s for HasCacheKey differs
+      # between instances, hence not a Struct.
+      class HasCacheKey
+        attr_reader :cache_key
+        def initialize(cache_key)
+          @cache_key = cache_key
+        end
+
+        def to_s
+          "HasCacheKey##{object_id}"
+        end
+      end
+
+      def cache_key
+        [
+          HasCacheKey.new(name),
+          HasCacheKey.new(id)
+        ]
+      end
+    end
+
     class UncachedAuthor < Author
       # To confirm cache_key is set using updated_at and cache_key option passed to cache
       undef_method :cache_key
@@ -106,6 +130,20 @@ module ActiveModelSerializers
       assert_equal(uncached_author_serializer.attributes.to_json, cache_store.fetch(key).to_json)
     end
 
+    def test_cache_key_expansion
+      author = AuthorWithExpandableCacheElements.new(id: 10, name: 'hello')
+      same_author = AuthorWithExpandableCacheElements.new(id: 10, name: 'hello')
+      diff_author = AuthorWithExpandableCacheElements.new(id: 11, name: 'hello')
+
+      author_serializer = AuthorSerializer.new(author)
+      same_author_serializer = AuthorSerializer.new(same_author)
+      diff_author_serializer = AuthorSerializer.new(diff_author)
+      adapter = AuthorSerializer.serialization_adapter_instance
+
+      assert_equal(author_serializer.cache_key(adapter), same_author_serializer.cache_key(adapter))
+      refute_equal(author_serializer.cache_key(adapter), diff_author_serializer.cache_key(adapter))
+    end
+
     def test_default_cache_key_fallback
       render_object_with_cache(@comment)
       key = "#{@comment.cache_key}/#{adapter.cache_key}"