Uses the agent ID instead of the unit ID when setting the cluster bind

addresses in the application data bag.

This is what is ultimately written to file and used by the agent, which
does knows its own ID, but not necessarily the unit's.

src/charm.py

@@ -171,12 +171,13 @@ def _on_dbcluster_relation_changed(self, event):
             # The event only has *other* units so include this
             # unit's bind address if we have managed to set it.
             ip = self._stored.db_bind_address
-            all_bind_addresses = {self.unit.name: ip} if ip else dict()
+            all_bind_addresses = {self._controller_agent_id(): ip} if ip else dict()
 
             for unit in relation.units:
                 unit_data = relation.data[unit]
                 if self.DB_BIND_ADDR_KEY in unit_data:
-                    all_bind_addresses[unit.name] = unit_data[self.DB_BIND_ADDR_KEY]
+                    agent_id = unit_data[self.AGENT_ID_KEY]
+                    all_bind_addresses[agent_id] = unit_data[self.DB_BIND_ADDR_KEY]
 
             if self._stored.all_bind_addresses == all_bind_addresses:
                 return

tests/test_charm.py

@@ -158,16 +158,21 @@ def test_dbcluster_relation_changed_single_addr(
         harness = self.harness
         mock_get_binding.return_value = mockBinding(['192.168.1.17'])
 
+        # This unit's agent ID happends to correspond with the unit ID.
         mock_get_agent_id.return_value = '0'
 
         harness.set_leader()
 
         # Have another unit enter the relation.
         # Its bind address should end up in the application data bindings list.
+        # Note that the agent ID doesn not correspond with the unit's ID
         relation_id = harness.add_relation('dbcluster', harness.charm.app)
         harness.add_relation_unit(relation_id, 'juju-controller/1')
         self.harness.update_relation_data(
-            relation_id, 'juju-controller/1', {'db-bind-address': '192.168.1.100'})
+            relation_id, 'juju-controller/1', {
+                'db-bind-address': '192.168.1.100',
+                'agent-id': '9',
+            })
 
         mock_reload_config.assert_called_once()
 
@@ -176,7 +181,7 @@ def test_dbcluster_relation_changed_single_addr(
         self.assertEqual(unit_data['agent-id'], '0')
 
         app_data = harness.get_relation_data(relation_id, 'juju-controller')
-        exp = {"juju-controller/0": "192.168.1.17", "juju-controller/1": "192.168.1.100"}
+        exp = {'0': '192.168.1.17', '9': '192.168.1.100'}
         self.assertEqual(json.loads(app_data['db-bind-addresses']), exp)
 
         harness.evaluate_status()