Tries to retrieve an isinstance of this model from the database
        given a value for a defined index. Return None in case of failure

def get_by(cls, field, value):
        ''' Tries to retrieve an isinstance of this model from the database
        given a value for a defined index. Return None in case of failure '''
        redis = cls.get_redis()
        key = cls.cls_key()+':index_'+field

        id = redis.hget(key, value)

        if id:
            return cls.get(debyte_string(id))

        return None