Update persistence.rb

compact is faster than looping with `reject` and achieves the same result

lib/active_graph/shared/persistence.rb

@@ -144,7 +144,7 @@ def destroyed?
 
     # @return [Hash] all defined and none nil properties
     def props
-      attributes.reject { |_, v| v.nil? }.symbolize_keys
+      attributes.compact.transform_keys(&:to_sym)
     end
 
     # @return true if the attributes hash has been frozen