Simplify directed graph example

Changed the "directed graph" example to no longer consider
integer identifiers of nodes as significant; the "higher" / "lower"
references now allow mutual edges in both directions.

Change-Id: Ibfd5b420f0451a6fc746f0bcbbbb062f8f88dc1d
Fixes: #3698
(cherry picked from commit c3de4061fd490adcd8b75c79685f4a831b869f9e)

2 files changed, 51 insertions, 47 deletions

diff --git a/doc/build/changelog/changelog_10.rst b/doc/build/changelog/changelog_10.rst
index f5de4f763..a30e3c567 100644
--- a/doc/build/changelog/changelog_10.rst
+++ b/doc/build/changelog/changelog_10.rst
@@ -19,6 +19,14 @@
     :version: 1.0.13
 
     .. change::
+        :tags: bug, examples
+        :tickets: 3698
+
+        Changed the "directed graph" example to no longer consider
+        integer identifiers of nodes as significant; the "higher" / "lower"
+        references now allow mutual edges in both directions.
+
+    .. change::
         :tags: bug, sql
         :tickets: 3690
 
diff --git a/examples/graphs/directed_graph.py b/examples/graphs/directed_graph.py
index b822cda89..7bcfc5683 100644
--- a/examples/graphs/directed_graph.py
+++ b/examples/graphs/directed_graph.py
@@ -1,57 +1,51 @@
 """a directed graph example."""
 
-from sqlalchemy import MetaData, Table, Column, Integer, ForeignKey, \
+from sqlalchemy import Column, Integer, ForeignKey, \
     create_engine
-from sqlalchemy.orm import mapper, relationship, sessionmaker
+from sqlalchemy.orm import relationship, sessionmaker
 from sqlalchemy.ext.declarative import declarative_base
 
 Base = declarative_base()
 
+
 class Node(Base):
     __tablename__ = 'node'
 
     node_id = Column(Integer, primary_key=True)
 
-    def __init__(self, id):
-        self.node_id = id
-
-    def add_neighbors(self, *nodes):
-        for node in nodes:
-            Edge(self, node)
-        return self
-
     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(Base):
     __tablename__ = 'edge'
 
-    lower_id = Column(Integer,
-                        ForeignKey('node.node_id'),
-                        primary_key=True)
+    lower_id = Column(
+        Integer,
+        ForeignKey('node.node_id'),
+        primary_key=True)
 
-    higher_id = Column(Integer,
-                        ForeignKey('node.node_id'),
-                        primary_key=True)
+    higher_id = Column(
+        Integer,
+        ForeignKey('node.node_id'),
+        primary_key=True)
 
-    lower_node = relationship(Node,
-                                primaryjoin=lower_id==Node.node_id,
-                                backref='lower_edges')
-    higher_node = relationship(Node,
-                                primaryjoin=higher_id==Node.node_id,
-                                backref='higher_edges')
+    lower_node = relationship(
+        Node,
+        primaryjoin=lower_id == Node.node_id,
+        backref='lower_edges')
+
+    higher_node = relationship(
+        Node,
+        primaryjoin=higher_id == Node.node_id,
+        backref='higher_edges')
 
-    # here we have lower.node_id <= higher.node_id
     def __init__(self, n1, n2):
-        if n1.node_id < n2.node_id:
-            self.lower_node = n1
-            self.higher_node = n2
-        else:
-            self.lower_node = n2
-            self.higher_node = n1
+        self.lower_node = n1
+        self.higher_node = n2
 
 engine = create_engine('sqlite://', echo=True)
 Base.metadata.create_all(engine)
@@ -59,29 +53,31 @@ Base.metadata.create_all(engine)
 session = sessionmaker(engine)()
 
 # create a directed graph like this:
-#       n1 -> n2 -> n5
+#       n1 -> n2 -> n1
+#                -> 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_neighbors(n5, n1)
-n3.add_neighbors(n6)
-n7.add_neighbors(n2)
-n1.add_neighbors(n3)
+n1 = Node()
+n2 = Node()
+n3 = Node()
+n4 = Node()
+n5 = Node()
+n6 = Node()
+n7 = Node()
+
+Edge(n1, n2)
+Edge(n1, n3)
+Edge(n2, n1)
+Edge(n2, n5)
+Edge(n2, n7)
+Edge(n3, n6)
 
 session.add_all([n1, n2, n3, n4, n5, n6, n7])
 session.commit()
 
-assert [x.node_id for x in n3.higher_neighbors()] == [6]
-assert [x.node_id for x in n3.lower_neighbors()] == [1]
-assert [x.node_id for x in n2.lower_neighbors()] == [1]
-assert [x.node_id for x in n2.higher_neighbors()] == [5,7]
-
+assert [x for x in n3.higher_neighbors()] == [n6]
+assert [x for x in n3.lower_neighbors()] == [n1]
+assert [x for x in n2.lower_neighbors()] == [n1]
+assert [x for x in n2.higher_neighbors()] == [n1, n5, n7]