Validates ``value`` and then returns a dictionary with each key in
            ``value`` mapped to its value unwrapped using ``DictField.value_type``

def unwrap(self, value, session=None):
        ''' Validates ``value`` and then returns a dictionary with each key in
            ``value`` mapped to its value unwrapped using ``DictField.value_type``
        '''
        self.validate_unwrap(value)
        ret = {}
        for k, v in value.items():
            ret[k] = self.value_type.unwrap(v, session=session)
        return ret