# apparently based on https://github.com/sqlalchemy/sqlalchemy/blob/rel_0_7/examples/beaker_caching/caching_query.py

"""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

    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 mapper option 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, query_cls=CachingQuery):

    def query(*arg, **kw):

        return query_cls(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 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.

    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

def _set_cache_parameters(query, region, namespace, cache_key):

    if hasattr(query, '_cache_parameters'):

        region, namespace, cache_key = query._cache_parameters

        raise ValueError("This query is already configured "

                        "for region %r namespace %r" %

                        (region, namespace)

                    )

    query._cache_parameters = region, safe_str(namespace), cache_key

class FromCache(MapperOption):

    """Specifies that a Query should load results from a cache."""

    propagate_to_loaders = False

    def __init__(self, region, namespace, cache_key=None):

        """Construct a new FromCache.

        :param region: the cache region.  Should be a

        region configured in the Beaker CacheManager.

        :param namespace: the cache namespace.  Should

        be a name uniquely describing the target Query's

        lexical structure.

        :param cache_key: optional.  A string cache key

        that will serve as the key to the query.   Use this

        if your query has a huge amount of parameters (such

        as when using in_()) which correspond more simply to

        some other identifier.

        """

        self.region = region

        self.namespace = namespace

        self.cache_key = cache_key

    def process_query(self, query):

        """Process a Query during normal loading operation."""

        _set_cache_parameters(query, self.region, self.namespace,

                              self.cache_key)

def _params_from_query(query):

    """Pull the bind parameter values from a query.

    This takes into account any scalar attribute bindparam set up.

    E.g. params_from_query(query.filter(Cls.foo==5).filter(Cls.bar==7)))

    would return [5, 7].

    """

    v = []

    def visit_bindparam(bind):

        if bind.key in query._params:

            value = query._params[bind.key]

        elif bind.callable:

            # lazyloader may dig a callable in here, intended

            # to late-evaluate params after autoflush is called.

            # convert to a scalar value.

            value = bind.callable()

        else:

            value = bind.value

        v.append(value)

    if query._criterion is not None:

        visitors.traverse(query._criterion, {}, {'bindparam': visit_bindparam})

    for f in query._from_obj:

        visitors.traverse(f, {}, {'bindparam': visit_bindparam})