From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Wed, 13 Jan 2010 20:42:09 +0000 (+0000)
Subject: local session caching example
X-Git-Tag: rel_0_6beta1~74
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=6e6b13b945299aede8bade024403d74af43279db;p=thirdparty%2Fsqlalchemy%2Fsqlalchemy.git

local session caching example
---

diff --git a/examples/beaker_caching/README b/examples/beaker_caching/README
index 1eb93f8734..702690036f 100644
--- a/examples/beaker_caching/README
+++ b/examples/beaker_caching/README
@@ -30,6 +30,8 @@ Three endpoint scripts, in order of complexity, are run as follows:
 
    python examples/beaker_caching/advanced.py
 
+   python examples/beaker_caching/local_session_caching.py
+
 
 Listing of files:
 
@@ -53,3 +55,8 @@ relation endpoints, so that lazyloads load from cache.
 
 advanced.py - Further examples of how to use FromCache.  Combines
 techniques from the first two scripts.
+
+local_session_caching.py - Grok everything so far ?   This example
+creates a new Beaker container that will persist data in a dictionary
+which is local to the current session.   remove() the session
+and the cache is gone.
\ No newline at end of file
diff --git a/examples/beaker_caching/local_session_caching.py b/examples/beaker_caching/local_session_caching.py
new file mode 100644
index 0000000000..c422600d87
--- /dev/null
+++ b/examples/beaker_caching/local_session_caching.py
@@ -0,0 +1,95 @@
+"""local_session_caching.py
+
+Create a new Beaker cache type + a local region that will store
+cached data local to the current Session.
+
+This is an advanced example which assumes familiarity
+with the basic operation of CachingQuery.
+
+"""
+
+from beaker import cache, container
+import collections
+
+class ScopedSessionNamespace(container.MemoryNamespaceManager):
+    """A Beaker cache type which will cache objects locally on 
+    the current session.
+
+    When used with the query_cache system, the effect is that the objects
+    in the cache are the same as that within the session - the merge()
+    is a formality that doesn't actually create a second instance.  
+    This makes it safe to use for updates of data from an identity
+    perspective (still not ideal for deletes though).
+
+    When the session is removed, the cache is gone too, so the cache
+    is automatically disposed upon session.remove().
+
+    """
+    
+    def __init__(self, namespace, scoped_session, **kwargs):
+        """__init__ is called by Beaker itself."""
+        
+        container.NamespaceManager.__init__(self, namespace)
+        self.scoped_session = scoped_session
+    
+    @classmethod
+    def create_session_container(cls, beaker_name, scoped_session):
+        """Create a new session container for a given scoped_session."""
+        
+        def create_namespace(namespace, **kw):
+            return cls(namespace, scoped_session, **kw)
+        cache.clsmap[beaker_name] = create_namespace
+        
+    @property
+    def dictionary(self):
+        """Return the cache dictionary used by this MemoryNamespaceManager."""
+        
+        sess = self.scoped_session()
+        try:
+            nscache = sess._beaker_cache
+        except AttributeError:
+            sess._beaker_cache = nscache = collections.defaultdict(dict)
+        return nscache[self.namespace]
+
+
+if __name__ == '__main__':
+    import __init__ # if running as a script
+    import meta
+    
+    # create a Beaker container type called "ext:local_session".
+    # it will reference the ScopedSession in meta.
+    ScopedSessionNamespace.create_session_container("ext:local_session", meta.Session)
+    
+    # set up a region based on this new container type.
+    meta.cache_manager.regions['local_session'] ={'type':'ext:local_session'}
+    
+    from model import Person
+    
+    # query to load Person by name, with criterion
+    # of "person 10"
+    q = meta.Session.query(Person).\
+                    options(meta.FromCache("local_session", "by_name")).\
+                    filter(Person.name=="person 10")
+                    
+    # load from DB
+    person10 = q.one()
+    
+    # next call, the query is cached.
+    person10 = q.one()
+
+    # clear out the Session.  The "_beaker_cache" dictionary
+    # disappears with it.
+    meta.Session.remove()
+    
+    # query calls from DB again
+    person10 = q.one()
+    
+    # identity is preserved - person10 is the *same* object that's
+    # ultimately inside the cache.   So it is safe to manipulate
+    # the not-queried-for attributes of objects when using such a 
+    # cache without the need to invalidate - however, any change 
+    # that would change the results of a cached query, such as 
+    # inserts, deletes, or modification to attributes that are 
+    # part of query criterion, still require careful invalidation.
+    cache, key = q._get_cache_plus_key()
+    assert person10 is cache.get(key)[0]