Fix undefined memoized_methods error raised when a parent class has not call memoize

lib/memoist.rb

@@ -4,6 +4,16 @@
 
 module Memoist
 
+  def self.extended(extender)
+    Memoist.memoist_eval(extender) do
+      unless singleton_class.method_defined?(:memoized_methods)
+        def self.memoized_methods
+          @_memoized_methods ||= []
+        end
+      end
+    end
+  end
+
   def self.memoized_ivar_for(method_name, identifier=nil)
     "@#{memoized_prefix(identifier)}_#{escape_punctuation(method_name)}"
   end
@@ -116,14 +126,6 @@ def memoize(*method_names)
       identifier = method_names.pop[:identifier]
     end
 
-    Memoist.memoist_eval(self) do
-      unless singleton_class.method_defined?(:memoized_methods)
-        def self.memoized_methods
-          @_memoized_methods ||= []
-        end
-      end
-    end
-
     method_names.each do |method_name|
       unmemoized_method = Memoist.unmemoized_method_for(method_name, identifier)
       memoized_ivar = Memoist.memoized_ivar_for(method_name, identifier)

test/memoist_test.rb

@@ -181,6 +181,33 @@ def counter
     memoize :counter
   end
 
+  class Abb
+    extend Memoist
+
+    def run(*args)
+      flush_cache if respond_to?(:flush_cache)
+      execute
+    end
+
+    def execute
+      some_method
+    end
+
+    def some_method
+      # Override this
+    end
+  end
+
+  class Bbb < Abb
+
+    def some_method
+      :foo
+    end
+    memoize :some_method
+  end
+
+
+
   def setup
     @person = Person.new
     @calculator = Calculator.new
@@ -256,6 +283,13 @@ def test_flush_cache
     assert_equal 2, @calculator.counter
   end
 
+  def test_flush_cache_in_child_class
+    x = Bbb.new
+
+    # This should not throw error
+    x.run
+  end
+
   def test_unmemoize_all
     assert_equal 1, @calculator.counter