(1) Both nodeId and portId in Rule can be NULL, (2) remove on delete cascade since rules should never mysteriously disappear from the rules table. If it let you delete a node with rules, that would be a UI or cleanup function bug.

controller/schema.sql

@@ -77,8 +77,8 @@ CREATE INDEX Relay_networkId ON Relay (networkId);
 CREATE TABLE Rule (
   networkId char(16) NOT NULL REFERENCES Network(id) ON DELETE CASCADE,
   ruleNo integer NOT NULL,
-  nodeId char(10) NOT NULL REFERENCES Node(id) ON DELETE CASCADE,
-  portId char(10) NOT NULL REFERENCES Node(id) ON DELETE CASCADE,
+  nodeId char(10) REFERENCES Node(id),
+  portId char(10) REFERENCES Node(id),
   vlanId integer,
   vlanPcp integer,
   etherType integer,

controller/schema.sql.c

@@ -78,8 +78,8 @@
 "CREATE TABLE Rule (\n"\
 "  networkId char(16) NOT NULL REFERENCES Network(id) ON DELETE CASCADE,\n"\
 "  ruleNo integer NOT NULL,\n"\
-"  nodeId char(10) NOT NULL REFERENCES Node(id) ON DELETE CASCADE,\n"\
-"  portId char(10) NOT NULL REFERENCES Node(id) ON DELETE CASCADE,\n"\
+"  nodeId char(10) REFERENCES Node(id),\n"\
+"  portId char(10) REFERENCES Node(id),\n"\
 "  vlanId integer,\n"\
 "  vlanPcp integer,\n"\
 "  etherType integer,\n"\