# cache_id, not used

    cache_id = Column("cache_id", Integer(), nullable=False, unique=True, default=None, primary_key=True)

    # cache_key as created by _get_cache_key

    cache_key = Column("cache_key", String(255, convert_unicode=False, assert_unicode=None), nullable=True, unique=None, default=None)

    # cache_args is a repo_name

    cache_args = Column("cache_args", String(255, convert_unicode=False, assert_unicode=None), nullable=True, unique=None, default=None)

    # instance sets cache_active True when it is caching,

    # other instances set cache_active to False to indicate that this cache is invalid

    cache_active = Column("cache_active", Boolean(), nullable=True, unique=None, default=False)

    def __init__(self, cache_key, repo_name=''):

        self.cache_key = cache_key

        self.cache_args = repo_name

        self.cache_active = False

    def __unicode__(self):

        return u"<%s('%s:%s')>" % (self.__class__.__name__,

                                  self.cache_id, self.cache_key)

    def get_prefix(self):

        """

        Guess prefix that might have been used in _get_cache_key to generate self.cache_key .

        Only used for informational purposes in repo_edit.html .

        """

    @classmethod

    def _get_cache_key(cls, key):

        """

        Wrapper for generating a unique cache key for this instance and "key".

        key must / will start with a repo_name which will be stored in .cache_args .

        """

        import rhodecode

        prefix = rhodecode.CONFIG.get('instance_id', '')

        return "%s%s" % (prefix, key)

    @classmethod

    def _get_or_create_inv_obj(cls, key, repo_name):

        inv_obj = Session().query(cls).filter(cls.cache_key == key).scalar()

        if not inv_obj:

            try:

                inv_obj = CacheInvalidation(key, repo_name)

                Session().add(inv_obj)

                Session().commit()

            except Exception:

                log.error(traceback.format_exc())

                Session().rollback()

        return inv_obj