diff --git a/ceilometer/storage/impl_mongodb.py b/ceilometer/storage/impl_mongodb.py
index c690d7479..7dd5d3606 100644
--- a/ceilometer/storage/impl_mongodb.py
+++ b/ceilometer/storage/impl_mongodb.py
@@ -616,6 +616,26 @@ class Connection(base.Connection):
                        for r in results['results']),
                       key=operator.attrgetter('period_start'))
 
+    @staticmethod
+    def _decode_matching_metadata(matching_metadata):
+        if isinstance(matching_metadata, dict):
+            #note(sileht): keep compatibility with old db format
+            return matching_metadata
+        else:
+            new_matching_metadata = {}
+            for elem in matching_metadata:
+                new_matching_metadata[elem['key']] = elem['value']
+            return new_matching_metadata
+
+    @staticmethod
+    def _encode_matching_metadata(matching_metadata):
+        if matching_metadata:
+            new_matching_metadata = []
+            for k, v in matching_metadata.iteritems():
+                new_matching_metadata.append({'key': k, 'value': v})
+            return new_matching_metadata
+        return matching_metadata
+
     def get_alarms(self, name=None, user=None,
                    project=None, enabled=True, alarm_id=None):
         """Yields a lists of alarms that match filters
@@ -636,6 +656,8 @@ class Connection(base.Connection):
             a = {}
             a.update(alarm)
             del a['_id']
+            a['matching_metadata'] = \
+                self._decode_matching_metadata(a['matching_metadata'])
             yield models.Alarm(**a)
 
     def update_alarm(self, alarm):
@@ -645,6 +667,9 @@ class Connection(base.Connection):
             # This is an insert, generate an id
             alarm.alarm_id = str(uuid.uuid1())
         data = alarm.as_dict()
+        data['matching_metadata'] = \
+            self._encode_matching_metadata(data['matching_metadata'])
+
         self.db.alarm.update(
             {'alarm_id': alarm.alarm_id},
             {'$set': data},
@@ -652,6 +677,8 @@ class Connection(base.Connection):
 
         stored_alarm = self.db.alarm.find({'alarm_id': alarm.alarm_id})[0]
         del stored_alarm['_id']
+        stored_alarm['matching_metadata'] = \
+            self._decode_matching_metadata(stored_alarm['matching_metadata'])
         return models.Alarm(**stored_alarm)
 
     def delete_alarm(self, alarm_id):
diff --git a/tests/storage/base.py b/tests/storage/base.py
index 096119e01..c870cfe37 100644
--- a/tests/storage/base.py
+++ b/tests/storage/base.py
@@ -801,16 +801,21 @@ class AlarmTest(DBTestBase):
                                'me', 'and-da-boys',
                                evaluation_periods=1,
                                period=60,
-                               alarm_actions=['http://nowhere/alarms']),
+                               alarm_actions=['http://nowhere/alarms'],
+                               matching_metadata={'key': 'value'}),
                   models.Alarm('orange-alert',
                                'test.fourty', 'gt', 75, 'avg',
                                'me', 'and-da-boys',
                                period=60,
-                               alarm_actions=['http://nowhere/alarms']),
+                               alarm_actions=['http://nowhere/alarms'],
+                               matching_metadata={'key2': 'value2'}),
                   models.Alarm('yellow-alert',
                                'test.five', 'lt', 10, 'min',
                                'me', 'and-da-boys',
-                               alarm_actions=['http://nowhere/alarms'])]
+                               alarm_actions=['http://nowhere/alarms'],
+                               matching_metadata=
+                               {'key2': 'value2',
+                                'user_metadata.key3': 'value3'})]
         for a in alarms:
             self.conn.update_alarm(a)
 
@@ -832,15 +837,23 @@ class AlarmTest(DBTestBase):
         self.assertEquals(yellow.state, models.Alarm.ALARM_INSUFFICIENT_DATA)
         self.assertEquals(yellow.ok_actions, [])
         self.assertEquals(yellow.insufficient_data_actions, [])
+        self.assertEquals(yellow.matching_metadata,
+                          {'key2': 'value2',
+                           'user_metadata.key3': 'value3'})
 
     def test_update(self):
         self.add_some_alarms()
         orange = list(self.conn.get_alarms(name='orange-alert'))[0]
         orange.enabled = False
         orange.state = models.Alarm.ALARM_INSUFFICIENT_DATA
+        orange.matching_metadata = {'new': 'value',
+                                    'user_metadata.new2': 'value4'}
         updated = self.conn.update_alarm(orange)
         self.assertEquals(updated.enabled, False)
         self.assertEquals(updated.state, models.Alarm.ALARM_INSUFFICIENT_DATA)
+        self.assertEquals(updated.matching_metadata,
+                          {'new': 'value',
+                           'user_metadata.new2': 'value4'})
 
     def test_update_llu(self):
         llu = models.Alarm('llu',
diff --git a/tests/storage/test_impl_mongodb.py b/tests/storage/test_impl_mongodb.py
index bff915064..6d73b168d 100644
--- a/tests/storage/test_impl_mongodb.py
+++ b/tests/storage/test_impl_mongodb.py
@@ -26,6 +26,8 @@
 
 import copy
 import datetime
+import uuid
+
 from oslo.config import cfg
 
 from tests.storage import base
@@ -33,6 +35,7 @@ from tests.storage import base
 from ceilometer.publisher import rpc
 from ceilometer import counter
 from ceilometer.storage import impl_mongodb
+from ceilometer.storage import models
 
 
 class MongoDBEngineTestBase(base.DBTestBase):
@@ -143,7 +146,25 @@ class StatisticsTest(base.StatisticsTest, MongoDBEngineTestBase):
 
 
 class AlarmTest(base.AlarmTest, MongoDBEngineTestBase):
-    pass
+    def prepare_old_matching_metadata_alarm(self):
+        alarm = models.Alarm('old-alert',
+                             'test.one', 'eq', 36, 'count',
+                             'me', 'and-da-boys',
+                             evaluation_periods=1,
+                             period=60,
+                             alarm_actions=['http://nowhere/alarms'],
+                             matching_metadata={'key': 'value'})
+        alarm.alarm_id = str(uuid.uuid1())
+        data = alarm.as_dict()
+        self.conn.db.alarm.update(
+            {'alarm_id': alarm.alarm_id},
+            {'$set': data},
+            upsert=True)
+
+    def test_alarm_get_old_matching_metadata_format(self):
+        self.prepare_old_matching_metadata_alarm()
+        old = list(self.conn.get_alarms(name='old-alert'))[0]
+        self.assertEquals(old.matching_metadata, {'key': 'value'})
 
 
 class CompatibilityTest(MongoDBEngineTestBase):