set Spawner.server directly

avoids Spawner.server property looking up on the ORM every time,
which is expensive and we want to check `Spawner.server is None` often.

jupyterhub/objects.py

@@ -62,6 +62,11 @@ class Server(HasTraits):
             return self.connect_port
         return self.port
 
+    @classmethod
+    def from_orm(cls, orm_server):
+        """Create a server from an orm.Server"""
+        return cls(orm_server=orm_server)
+
     @classmethod
     def from_url(cls, url):
         """Create a Server from a given URL"""

jupyterhub/services/service.py

@@ -235,7 +235,7 @@ class Service(LoggingConfigurable):
     @property
     def server(self):
         if self.orm.server:
-            return Server(orm_server=self.orm.server)
+            return Server.from_orm(self.orm.server)
         else:
             return None
 

jupyterhub/spawner.py

@@ -15,13 +15,15 @@ import warnings
 from subprocess import Popen
 from tempfile import mkdtemp
 
+from sqlalchemy import inspect
+
 from tornado import gen
 from tornado.ioloop import PeriodicCallback, IOLoop
 
 from traitlets.config import LoggingConfigurable
 from traitlets import (
     Any, Bool, Dict, Instance, Integer, Float, List, Unicode,
-    validate,
+    observe, validate,
 )
 
 from .objects import Server
@@ -89,6 +91,14 @@ class Spawner(LoggingConfigurable):
     authenticator = Any()
     hub = Any()
     orm_spawner = Any()
+
+    @observe('orm_spawner')
+    def _orm_spawner_changed(self, change):
+        if change.new and change.new.server:
+            self._server = Server(orm_server=change.new.server)
+        else:
+            self._server = None
+
     user = Any()
 
     def __init_subclass__(cls, **kwargs):
@@ -105,8 +115,24 @@ class Spawner(LoggingConfigurable):
 
     @property
     def server(self):
+        if hasattr(self, '_server'):
+            return self._server
         if self.orm_spawner and self.orm_spawner.server:
             return Server(orm_server=self.orm_spawner.server)
+    
+    @server.setter
+    def server(self, server):
+        self._server = server
+        if self.orm_spawner:
+            if self.orm_spawner.server is not None:
+                # delete the old value
+                db = inspect(self.orm_spawner.server).session
+                db.delete(self.orm_spawner.server)
+            if server is None:
+                self.orm_spawner.server = None
+            else:
+                self.orm_spawner.server = server.orm_server
+
     @property
     def name(self):
         if self.orm_spawner:

jupyterhub/tests/test_spawner.py

@@ -16,7 +16,7 @@ import pytest
 from tornado import gen
 
 from ..user import User
-from ..objects import Hub
+from ..objects import Hub, Server
 from .. import spawner as spawnermod
 from ..spawner import LocalProcessSpawner, Spawner
 from .. import orm
@@ -234,7 +234,10 @@ def test_shell_cmd(db, tmpdir, request):
         cmd=[sys.executable, '-m', 'jupyterhub.tests.mocksu'],
         shell_cmd=['bash', '--rcfile', str(f), '-i', '-c'],
     )
-    s.orm_spawner.server = orm.Server()
+    server = orm.Server()
+    db.add(server)
+    db.commit()
+    s.server = Server.from_orm(server)
     db.commit()
     (ip, port) = yield s.start()
     request.addfinalizer(s.stop)

jupyterhub/user.py

@@ -9,12 +9,13 @@ from oauth2.error import ClientNotFoundError
 from sqlalchemy import inspect
 from tornado import gen
 from tornado.log import app_log
+from traitlets import HasTraits, Any, Dict, default
 
 from .utils import url_path_join, default_server_name
 
 from . import orm
 from ._version import _check_version, __version__
-from traitlets import HasTraits, Any, Dict, observe, default
+from .objects import Server
 from .spawner import LocalProcessSpawner
 from .crypto import encrypt, decrypt, CryptKeeper, EncryptionUnavailable, InvalidToken
 
@@ -314,8 +315,8 @@ class User(HasTraits):
 
 
         spawner = self.spawners[server_name]
-        spawner.server = Server(orm_server=orm_server)
+        spawner.server = server = Server(orm_server=orm_server)
-        assert orm_spawner.server is orm_server
+        assert spawner.orm_spawner.server is orm_server
 
         # Passing user_options to the spawner
         spawner.user_options = options or {}
@@ -452,8 +453,6 @@ class User(HasTraits):
             spawner.orm_spawner.state = spawner.get_state()
             self.last_activity = datetime.utcnow()
             # remove server entry from db
-            if spawner.server is not None:
-                self.db.delete(spawner.orm_spawner.server)
             spawner.server = None
             if not spawner.will_resume:
                 # find and remove the API token if the spawner isn't