Use a joined relationship for AZ info on networks

The previous code was doing a DB lookup for each network's
availability zone which was significantly impacting the
performance of network listings.

This patch adjusts the network model to be automatically joined
to the DHCP agents table that the AZ code uses to populate the
AZs for the network.

Change-Id: I908ceb1a68e0eed7c304e3ff82279ad6fa406167
Closes-Bug: #1525740

neutron/db/agentschedulers_db.py

@@ -464,15 +464,9 @@ class AZDhcpAgentSchedulerDbMixin(DhcpAgentSchedulerDbMixin,
                                   network_az.NetworkAvailabilityZoneMixin):
     """Mixin class to add availability_zone supported DHCP agent scheduler."""
 
-    def get_network_availability_zones(self, network_id):
-        context = ncontext.get_admin_context()
-        with context.session.begin():
-            query = context.session.query(agents_db.Agent.availability_zone)
-            query = query.join(NetworkDhcpAgentBinding)
-            query = query.filter(
-                NetworkDhcpAgentBinding.network_id == network_id)
-            query = query.group_by(agents_db.Agent.availability_zone)
-            return [item[0] for item in query]
+    def get_network_availability_zones(self, network):
+        zones = {agent.availability_zone for agent in network.dhcp_agents}
+        return list(zones)
 
 
 # helper functions for readability.

neutron/db/availability_zone/network.py

@@ -29,7 +29,7 @@ class NetworkAvailabilityZoneMixin(net_az.NetworkAvailabilityZonePluginBase):
         net_res[az_ext.AZ_HINTS] = az_ext.convert_az_string_to_list(
             net_db[az_ext.AZ_HINTS])
         net_res[az_ext.AVAILABILITY_ZONES] = (
-            self.get_network_availability_zones(net_db['id']))
+            self.get_network_availability_zones(net_db))
 
     common_db_mixin.CommonDbMixin.register_dict_extend_funcs(
         attributes.NETWORKS, ['_extend_availability_zone'])

neutron/db/models_v2.py

@@ -267,3 +267,5 @@ class Network(model_base.HasStandardAttributes, model_base.BASEV2,
                                     backref='network', lazy='joined',
                                     cascade='all, delete, delete-orphan')
     availability_zone_hints = sa.Column(sa.String(255))
+    dhcp_agents = orm.relationship('Agent', lazy='joined', viewonly=True,
+                                   secondary='networkdhcpagentbindings')

neutron/extensions/network_availability_zone.py

@@ -64,5 +64,5 @@ class Network_availability_zone(extensions.ExtensionDescriptor):
 class NetworkAvailabilityZonePluginBase(object):
 
     @abc.abstractmethod
-    def get_network_availability_zones(self, network_id):
+    def get_network_availability_zones(self, network):
         """Return availability zones which a network belongs to"""