"""caching_query.py

Represent persistence structures which allow the usage of

Beaker caching with SQLAlchemy.

The three new concepts introduced here are:

 * CachingQuery - a Query subclass that caches and

   retrieves results in/from Beaker.

 * FromCache - a query option that establishes caching

   parameters on a Query

 * _params_from_query - extracts value parameters from

   a Query.

The rest of what's here are standard SQLAlchemy and

Beaker constructs.

"""

import beaker

from beaker.exceptions import BeakerException

from sqlalchemy.orm.interfaces import MapperOption

from sqlalchemy.orm.query import Query

from sqlalchemy.sql import visitors

from kallithea.lib.utils2 import safe_str

class CachingQuery(Query):

    """A Query subclass which optionally loads full results from a Beaker

    cache region.

    The CachingQuery stores additional state that allows it to consult

    a Beaker cache before accessing the database:

    * A "region", which is a cache region argument passed to a

      Beaker CacheManager, specifies a particular cache configuration

      (including backend implementation, expiration times, etc.)

    * A "namespace", which is a qualifying name that identifies a

      group of keys within the cache.  A query that filters on a name

      might use the name "by_name", a query that filters on a date range

      to a joined table might use the name "related_date_range".

    When the above state is present, a Beaker cache is retrieved.

    The "namespace" name is first concatenated with

    a string composed of the individual entities and columns the Query

    requests, i.e. such as ``Query(User.id, User.name)``.