Remove ORM relationship between ports and networks

This relationship doesn't serve a purpose and it results
in SQLAlchemy trying to set the network_id column on ports
to NULL when attempting to delete a network with ports in it.

This results in the following exception:

IntegrityError: (1048, u"Column 'network_id' cannot be null")

This exception is not treated as a DBReferenceError by olso.db
which makes detecting races between ports being created on a
network being deleted very noisy in the logs since oslo.db treats
this as a generic DBError.

By removing the relationship, deleting a network will now just
result in a boring standard foreign key constraint failure which
will propagate from oslo.db as a DBReferenceError that is easy
to catch.

The patch that depends on this one adjusts the delete network logic
to capture these ReferenceErrors.

Change-Id: I4166a9bbaded57a77576400929a150123b8f1b2e

neutron/db/models_v2.py

@@ -236,7 +236,6 @@ class Network(standard_attr.HasStandardAttributes, model_base.BASEV2,
     """Represents a v2 neutron network."""
 
     name = sa.Column(sa.String(db_const.NAME_FIELD_SIZE))
-    ports = orm.relationship(Port, backref='networks')
     subnets = orm.relationship(
         Subnet, backref=orm.backref('networks', lazy='subquery'),
         lazy="subquery")

neutron/tests/unit/db/test_provisioning_blocks.py

@@ -43,9 +43,10 @@ class TestStatusBarriers(testlib_api.SqlTestCase):
     def _make_port(self):
         net = self._make_net()
         with self.ctx.session.begin():
-            port = models_v2.Port(networks=net, mac_address='1', tenant_id='1',
-                                  admin_state_up=True, status='DOWN',
-                                  device_id='2', device_owner='3')
+            port = models_v2.Port(network_id=net.id, mac_address='1',
+                                  tenant_id='1', admin_state_up=True,
+                                  status='DOWN', device_id='2',
+                                  device_owner='3')
             self.ctx.session.add(port)
         return port
 

neutron/tests/unit/plugins/ml2/test_db.py

@@ -399,6 +399,7 @@ class Ml2DvrDBTestCase(testlib_api.SqlTestCase):
     def test_distributed_port_binding_deleted_by_port_deletion(self):
         with self.ctx.session.begin(subtransactions=True):
             self.ctx.session.add(models_v2.Network(id='network_id'))
+        with self.ctx.session.begin(subtransactions=True):
             device_owner = constants.DEVICE_OWNER_DVR_INTERFACE
             port = models_v2.Port(
                 id='port_id',