Avoid to hits the database too many times to check if the list of

printers needs to be refreshed.

Keep the last update datetime in cache and invalidate this datetime if is
is older than POLL_INTERVAL.  Thus, one process won't hit the DB more
than 1 time every POLL_INTERVAL (10 seconds currently) to check if it
needs to update the list.

base_report_to_printer/printing.py

@@ -33,6 +33,7 @@ import cups
 import psycopg2
 
 from openerp import models, fields, api, sql_db
+from openerp.tools import ormcache
 
 _logger = logging.getLogger(__name__)
 
@@ -125,18 +126,45 @@ class PrintingPrinterPolling(models.Model):
         if locked:
             polling.write({'last_update': fields.Datetime.now()})
 
-    @api.model
-    def need_update(self):
+    @ormcache()
+    def _last_update_cached(self):
+        """ Get the last update's datetime, the returned value is cached """
         polling = self.find_unique_record()
         if not polling:
-            return True
+            return False
         last_update = polling.last_update
         if last_update:
             last_update = fields.Datetime.from_string(last_update)
+        return last_update
+
+    @api.model
+    def last_update_cached(self):
+        """ Returns the last update datetime from a cache
+
+        The check if the list of printers needs to be refreshed is
+        called very often (each time a browse is done on ``res.users``),
+        so we avoid to hit the database on every updates by keeping the
+        last value in cache.
+        The cache has no expiration so we manually clear it when the
+        poll interval (defaulted to 10 seconds) is reached.
+        """
+        last_update = self._last_update_cached()
+        now = datetime.now()
+        if last_update and (now - last_update).seconds >= POLL_INTERVAL:
+            # Invalidates last_update_cached and read a fresh value
+            # from the database
+            self.clear_caches()
+            return self._last_update_cached()
+        return last_update
+
+    @api.model
+    def need_update(self):
+        last_update = self.last_update_cached()
         now = datetime.now()
         # Only update printer status if current status is more than 10
         # seconds old.
         if not last_update or (now - last_update).seconds >= POLL_INTERVAL:
+            self.clear_caches()  # invalidates last_update_cached
             return True
         return False