Make dummy probe use tighter pattern for random readings

to make for a more reasonable-looking graph

rattail_tempmon/db/model.py

@@ -297,6 +297,16 @@ class Probe(Base):
     def __str__(self):
         return self.description
 
+    def last_reading(self):
+        """
+        Returns the reading which was taken most recently for this probe.
+        """
+        session = orm.object_session(self)
+        return session.query(Reading)\
+                      .filter(Reading.probe == self)\
+                      .order_by(Reading.taken.desc())\
+                      .first()
+
     def start_status(self, status, time):
         """
         Update the "started" timestamp field for the given status.  This is
@@ -413,6 +423,7 @@ class Reading(Base):
         """,
         backref=orm.backref(
             'readings',
+            order_by='Reading.taken',
             cascade='all, delete-orphan'))
 
     taken = sa.Column(sa.DateTime(), nullable=False, default=datetime.datetime.utcnow)