Add rattail purge-tempmon command

for getting rid of all those old temperature readings, since
they're mostly just useful in real-time..

rattail_tempmon/commands.py

@@ -26,7 +26,16 @@ Tempmon commands
 
 from __future__ import unicode_literals, absolute_import
 
+import datetime
+import logging
+
 from rattail import commands
+from rattail.time import localtime, make_utc
+
+from rattail_tempmon.db import Session as TempmonSession, model as tempmon
+
+
+log = logging.getLogger(__name__)
 
 
 class ExportHotCooler(commands.ImportSubcommand):
@@ -38,6 +47,46 @@ class ExportHotCooler(commands.ImportSubcommand):
     handler_spec = 'rattail_tempmon.hotcooler.importing.tempmon:FromTempmonToHotCooler'
 
 
+class PurgeTempmon(commands.Subcommand):
+    """
+    Purge stale data from Tempmon database
+    """
+    name = 'purge-tempmon'
+    description = __doc__.strip()
+
+    def add_parser_args(self, parser):
+        parser.add_argument('--keep', metavar='DAYS', required=True, type=int,
+                            help="Number of days for which data should be kept.")
+        parser.add_argument('--dry-run', action='store_true',
+                            help="Go through the full motions and allow logging etc. to "
+                            "occur, but rollback (abort) the transaction at the end.")
+
+    def run(self, args):
+        cutoff = localtime(self.config).date() - datetime.timedelta(days=args.keep)
+        cutoff = localtime(self.config, datetime.datetime.combine(cutoff, datetime.time(0)))
+        session = TempmonSession()
+
+        readings = session.query(tempmon.Reading)\
+                          .filter(tempmon.Reading.taken < make_utc(cutoff))
+        count = readings.count()
+
+        def purge(reading, i):
+            session.delete(reading)
+            if i % 200 == 0:
+                session.flush()
+
+        self.progress_loop(purge, readings, count=count, message="Purging stale readings")
+        log.info("deleted {} stale readings".format(count))
+
+        if args.dry_run:
+            session.rollback()
+            log.info("dry run, so transaction was rolled back")
+        else:
+            session.commit()
+            log.info("transaction was committed")
+        session.close()
+
+
 class TempmonClient(commands.Subcommand):
     """
     Manage the tempmon-client daemon

setup.py

@@ -99,6 +99,7 @@ setup(
     entry_points = {
         'rattail.commands': [
             'export-hotcooler = rattail_tempmon.commands:ExportHotCooler',
+            'purge-tempmon = rattail_tempmon.commands:PurgeTempmon',
             'tempmon-client = rattail_tempmon.commands:TempmonClient',
             'tempmon-server = rattail_tempmon.commands:TempmonServer',
         ],