"""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

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)``.

    The Beaker cache is then loaded from the cache manager based

            'Check if proper cache settings are in the .ini files' % region)

    kw = beaker.cache.cache_regions[region]

    return beaker.cache.Cache._get_cache(name, kw)

def _get_cache_parameters(query):

    """For a query with cache_region and cache_namespace configured,

    return the corresponding Cache instance and cache key, based

    on this query's current criterion and parameter values.

    """

    if not hasattr(query, '_cache_parameters'):

        raise ValueError("This Query does not have caching "

                         "parameters configured.")

    region, namespace, cache_key = query._cache_parameters

    namespace = _namespace_from_query(namespace, query)

    if cache_key is None:

        # cache key - the value arguments from this query's parameters.

        args = _params_from_query(query)

        args.append(query._limit)

        args.append(query._offset)

    if cache_key is None:

        raise Exception('Cache key cannot be None')

    # get cache

    #cache = query.cache_manager.get_cache_region(namespace, region)

    cache = get_cache_region(namespace, region)

    # optional - hash the cache_key too for consistent length

    # import uuid

    # cache_key= str(uuid.uuid5(uuid.NAMESPACE_DNS, cache_key))

    return cache, cache_key

def _namespace_from_query(namespace, query):

    # cache namespace - the token handed in by the

    # option + class we're querying against

    namespace = " ".join([namespace] + [str(x) for x in query._entities])

    # memcached wants this

    namespace = namespace.replace(' ', '_')

    return namespace