mojikun_test.rb

def redefine_constant(full_const_name, value)
  parent_class, const_name = *parse_constant_namespace(full_const_name)
  orig = parent_class.const_get(const_name).dup

  parent_class.send(:remove_const, const_name)
  parent_class.const_set(const_name, value)

  yield

  parent_class.send(:remove_const, const_name)
  parent_class.const_set(const_name, orig)
end

def parse_constant_namespace(full_const_name)
  match = full_const_name.match(/\A((?<parent_class>.+)::)?(?<const_name>[^:]+)\z/)
  parent_class = match[:parent_class] ? eval(match[:parent_class]) : Object
  const_name = match[:const_name]

  [parent_class, const_name]
end