"""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

 * RelationshipCache - a variant of FromCache which is specific

   to a query invoked during a lazy load.

 * _params_from_query - extracts value parameters from 

   a Query.

The rest of what's here are standard SQLAlchemy and

Beaker constructs.

"""

from beaker.exceptions import BeakerException

from sqlalchemy.orm.interfaces import MapperOption

from sqlalchemy.orm.query import Query

from sqlalchemy.sql import visitors

import beaker

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

    on the region and composed namespace.  The key within the cache

    itself is then constructed against the bind parameters specified

    by this query, which are usually literals defined in the 

    WHERE clause.

    The FromCache and RelationshipCache mapper options below represent

    the "public" method of configuring this state upon the CachingQuery.

    """

    def __init__(self, manager, *args, **kw):

        self.cache_manager = manager

        Query.__init__(self, *args, **kw)

    def __iter__(self):

        """override __iter__ to pull results from Beaker

           if particular attributes have been configured.

           Note that this approach does *not* detach the loaded objects from

           the current session. If the cache backend is an in-process cache

           (like "memory") and lives beyond the scope of the current session's

           transaction, those objects may be expired. The method here can be

           modified to first expunge() each loaded item from the current

           session before returning the list of items, so that the items

           in the cache are not the same ones in the current Session.

        """

        if hasattr(self, '_cache_parameters'):

            return self.get_value(createfunc=lambda: list(Query.__iter__(self)))

        else:

            return Query.__iter__(self)

    def invalidate(self):

        """Invalidate the value represented by this Query."""

        cache, cache_key = _get_cache_parameters(self)

        cache.remove(cache_key)

    def get_value(self, merge=True, createfunc=None):

        """Return the value from the cache for this query.

        Raise KeyError if no value present and no

        createfunc specified.

        """

        cache, cache_key = _get_cache_parameters(self)

        ret = cache.get_value(cache_key, createfunc=createfunc)

        if merge:

            ret = self.merge_result(ret, load=False)

        return ret

    def set_value(self, value):

        """Set the value in the cache for this query."""

        cache, cache_key = _get_cache_parameters(self)

        cache.put(cache_key, value)

def query_callable(manager):

    def query(*arg, **kw):

        return CachingQuery(manager, *arg, **kw)

    return query

def get_cache_region(name, region):

    if region not in beaker.cache.cache_regions:

        raise BeakerException('Cache region `%s` not configured '

            '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 correspoinding 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)

        cache_key = " ".join([str(x) for x in args])

    # 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(' ', '_')