added graphing example

diff --git a/examples/graphs/graph1.py b/examples/graphs/graph1.py
new file mode 100644 (file)
index 0000000..af75d0f
--- /dev/null
+++ b/examples/graphs/graph1.py
@@ -0,0 +1,85 @@
+"""a directed graph example."""
+
+from sqlalchemy import *
+
+import logging
+logging.basicConfig()
+logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)
+
+meta = BoundMetaData('sqlite://')
+
+nodes = Table('nodes', meta,
+    Column("nodeid", Integer, primary_key=True)
+)
+ 
+# here we have lower.nodeid <= higher.nodeid
+edges = Table('edges', meta,
+    Column("lower_id", Integer, ForeignKey('nodes.nodeid'), primary_key=True),
+    Column("higher_id", Integer, ForeignKey('nodes.nodeid'), primary_key=True)
+)
+
+meta.create_all()
+
+class Node(object):
+    def __init__(self, id):
+        self.nodeid = id
+    def add_neighbor(self, othernode):
+        Edge(self, othernode)
+    def higher_neighbors(self):
+        return [x.higher_node for x in self.lower_edges]
+    def lower_neighbors(self):
+        return [x.lower_node for x in self.higher_edges]
+
+class Edge(object):
+    def __init__(self, n1, n2):
+        if n1.nodeid < n2.nodeid:
+            self.lower_node = n1
+            self.higher_node = n2
+        else:
+            self.lower_node = n2
+            self.higher_node = n1
+
+mapper(Node, nodes)
+mapper(Edge, edges, properties={
+    'lower_node':relation(Node,
+primaryjoin=edges.c.lower_id==nodes.c.nodeid, backref='lower_edges'),
+    'higher_node':relation(Node,
+primaryjoin=edges.c.higher_id==nodes.c.nodeid, backref='higher_edges')
+    }
+)
+
+session = create_session()
+
+# create a directed graph like this:
+#       n1 -> n2 -> n5
+#                -> n7
+#          -> n3 -> n6
+                
+n1 = Node(1)
+n2 = Node(2)
+n3 = Node(3)
+n4 = Node(4)
+n5 = Node(5)
+n6 = Node(6)
+n7 = Node(7)
+
+n2.add_neighbor(n5)
+n3.add_neighbor(n6)
+n7.add_neighbor(n2)
+n1.add_neighbor(n3)
+n2.add_neighbor(n1)
+
+[session.save(x) for x in [n1, n2, n3, n4, n5, n6, n7]]
+session.flush()
+
+session.clear()
+
+n2 = session.query(Node).get(2)
+n3 = session.query(Node).get(3)
+
+assert [x.nodeid for x in n3.higher_neighbors()] == [6]
+assert [x.nodeid for x in n3.lower_neighbors()] == [1]
+assert [x.nodeid for x in n2.lower_neighbors()] == [1]
+assert [x.nodeid for x in n2.higher_neighbors()] == [5,7]
+
+