A request comes to tornado's GET handler of a web app.
From the `GET` function, a `blocking_task` function is called. This `blocking_task` function has `@run_on_executor` decorator.

But this execution fails.
Could you please help on this. It seems that motor db is not able to execute the thread.

```
import time
from concurrent.futures import ThreadPoolExecutor
from tornado import gen, web
from tornado.concurrent import run_on_executor
from tornado.ioloop import IOLoop
import argparse
from common.config import APIConfig
import sys
import os
import motor

parser = argparse.ArgumentParser()
parser.add_argument("-c", "--config-file", dest='config_file',
                    help="Config file location")
args = parser.parse_args()
CONF = APIConfig().parse(args.config_file)

client = motor.MotorClient(CONF.mongo_url)
db = client[CONF.mongo_dbname]
class Handler(web.RequestHandler):

    executor = ThreadPoolExecutor(10)

    def initialize(self):
        """ Prepares the database for the entire class """
        self.db = self.settings["db"]

    @gen.coroutine
    def get(self):
        self.blocking_task()

    @run_on_executor
    def blocking_task(self):
        mongo_dict = self.db.test_cases.find_one({"name": "Ping"})

if __name__ == "__main__":
    app = web.Application([
        (r"/", Handler),
    ],
        db=db,
        debug=CONF.api_debug_on,
    )
    app.listen(8888)
    IOLoop.current().start()

> ERROR:tornado.application:Exception in callback <functools.partial
> object at 0x7f72dfbe48e8> Traceback (most recent call last):   File
> "/usr/local/lib/python2.7/dist-packages/tornado-4.3-py2.7-linux-x86_64.egg/tornado/ioloop.py",
> line 600, in _run_callback
>     ret = callback()   File "/usr/local/lib/python2.7/dist-packages/tornado-4.3-py2.7-linux-x86_64.egg/tornado/stack_context.py",
> line 275, in null_wrapper
>     return fn(*args, **kwargs)   File "/usr/local/lib/python2.7/dist-packages/motor-0.5-py2.7.egg/motor/frameworks/tornado.py",
> line 231, in callback
>     child_gr.switch(future.result()) error: cannot switch to a different thread

```

Could you please help on this.