1 files changed, 112 insertions, 0 deletions
diff --git a/lib/sqlalchemy/orm/shard.py b/lib/sqlalchemy/orm/shard.py
new file mode 100644
index 000000000..cc13f8c1f
--- /dev/null
+++ b/lib/sqlalchemy/orm/shard.py
@@ -0,0 +1,112 @@
+from sqlalchemy.orm.session import Session
+from sqlalchemy.orm import Query
+
+class ShardedSession(Session):
+    def __init__(self, shard_chooser, id_chooser, query_chooser, **kwargs):
+        """construct a ShardedSession.
+        
+            shard_chooser
+                a callable which, passed a Mapper and a mapped instance, returns a
+                shard ID.  this id may be based off of the attributes present within the
+                object, or on some round-robin scheme.  If the scheme is based on a
+                selection, it should set whatever state on the instance to mark it in
+                the future as participating in that shard.
+            
+            id_chooser
+                a callable, passed a tuple of identity values, which should return
+                a list of shard ids where the ID might reside.  The databases will
+                be queried in the order of this listing.
+                
+            query_chooser
+                for a given Query, returns the list of shard_ids where the query
+                should be issued.  Results from all shards returned will be 
+                combined together into a single listing.
+        
+        """
+        super(ShardedSession, self).__init__(**kwargs)
+        self.shard_chooser = shard_chooser
+        self.id_chooser = id_chooser
+        self.query_chooser = query_chooser
+        self.__binds = {}
+        self._mapper_flush_opts = {'connection_callable':self.connection}
+        self._query_cls = ShardedQuery
+        
+    def connection(self, mapper=None, instance=None, shard_id=None, **kwargs):
+        if shard_id is None:
+            shard_id = self.shard_chooser(mapper, instance)
+
+        if self.transaction is not None:
+            return self.transaction.connection(mapper, shard_id=shard_id)
+        else:
+            return self.get_bind(mapper, shard_id=shard_id, instance=instance).contextual_connect(**kwargs)
+    
+    def get_bind(self, mapper, shard_id=None, instance=None):
+        if shard_id is None:
+            shard_id = self.shard_chooser(mapper, instance)
+        return self.__binds[shard_id]
+
+    def bind_shard(self, shard_id, bind):
+        self.__binds[shard_id] = bind
+
+class ShardedQuery(Query):
+    def __init__(self, *args, **kwargs):
+        super(ShardedQuery, self).__init__(*args, **kwargs)
+        self.id_chooser = self.session.id_chooser
+        self.query_chooser = self.session.query_chooser
+        self._shard_id = None
+        
+    def _clone(self):
+        q = ShardedQuery.__new__(ShardedQuery)
+        q.__dict__ = self.__dict__.copy()
+        return q
+    
+    def set_shard(self, shard_id):
+        """return a new query, limited to a single shard ID.
+        
+        all subsequent operations with the returned query will 
+        be against the single shard regardless of other state.
+        """
+        
+        q = self._clone()
+        q._shard_id = shard_id
+        return q
+        
+    def _execute_and_instances(self, statement):
+        if self._shard_id is not None:
+            result = self.session.connection(mapper=self.mapper, shard_id=self._shard_id).execute(statement, **self._params)
+            try:
+                return iter(self.instances(result))
+            finally:
+                result.close()
+        else:
+            partial = []
+            for shard_id in self.query_chooser(self):
+                result = self.session.connection(mapper=self.mapper, shard_id=shard_id).execute(statement, **self._params)
+                try:
+                    partial = partial + list(self.instances(result))
+                finally:
+                    result.close()
+            # if some kind of in memory 'sorting' were done, this is where it would happen
+            return iter(partial)
+
+    def get(self, ident, **kwargs):
+        if self._shard_id is not None:
+            return super(ShardedQuery, self).get(ident)
+        else:
+            for shard_id in self.id_chooser(ident):
+                o = self.set_shard(shard_id).get(ident, **kwargs)
+                if o is not None:
+                    return o
+            else:
+                return None
+    
+    def load(self, ident, **kwargs):
+        if self._shard_id is not None:
+            return super(ShardedQuery, self).load(ident)
+        else:
+            for shard_id in self.id_chooser(ident):
+                o = self.set_shard(shard_id).load(ident, raiseerr=False, **kwargs)
+                if o is not None:
+                    return o
+            else:
+                raise exceptions.InvalidRequestError("No instance found for identity %s" % repr(ident))