peewee.OperationalError: Connection already opened. (peewee 3.3.4)
i found problem
it was when i tried to "create" record , and because of "Not Availabe ForeignKey" , peewee raised exception ,, and left connection open , till restarting workers , it return error ,, 

for solving this problem , we can use "Try , Except" , and "abort" request when this happen ,, or use gevent workers  It sounds like you're opening an already-opened connection. You will want to add middleware to manage the connections: http://docs.peewee-orm.com/en/latest/peewee/database.html#flask

Peewee stores connection state in a thread-local so you can safely use peewee with multi-threaded (or gevent) web apps.

I'd suggest that you post the relevant code on stackoverflow if you have more questions. @coleifer 
i did exactly same thing ,  http://docs.peewee-orm.com/en/latest/peewee/database.html#flask
peewee exceptions cause workers crash ,and  leave connection open,. Can you share the full traceback or some actual code? ```
ERROR in app: Exception on /course/form-save [POST]
Traceback (most recent call last):
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask_cors/extension.py", line 161, in wrapped_function
    return cors_after_request(app.make_response(f(*args, **kwargs)))
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/views.py", line 88, in view
    return self.dispatch_request(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flasgger/marshmallow_apispec.py", line 79, in dispatch_request
    return super(SwaggerView, self).dispatch_request(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/views.py", line 158, in dispatch_request
    return meth(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/app/course/api.py", line 131, in post
    Parse(data)
  File "/home/user/Workspace_Py/my_app/app/course/parser.py", line 10, in Parse
    Obj['course_price']=int(Obj['course_price'])
ValueError: invalid literal for int() with base 10: 'dsvvgfbgf'
[2018-07-01 13:00:10,859] ERROR in app: Exception on /course/form-save [POST]
Traceback (most recent call last):
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask_cors/extension.py", line 161, in wrapped_function
    return cors_after_request(app.make_response(f(*args, **kwargs)))
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/views.py", line 88, in view
    return self.dispatch_request(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flasgger/marshmallow_apispec.py", line 79, in dispatch_request
    return super(SwaggerView, self).dispatch_request(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/views.py", line 158, in dispatch_request
    return meth(*args, **kwargs)
  File "/home/user/Workspace_Py/my_app/app/course/api.py", line 131, in post
    Parse(data)
  File "/home/user/Workspace_Py/my_app/app/course/parser.py", line 10, in Parse
    Obj['course_price']=int(Obj['course_price'])
ValueError: invalid literal for int() with base 10: 'dsvvgfbgf'
[2018-07-01 13:00:17,098] ERROR in app: Exception on /auth/check [POST]
Traceback (most recent call last):
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask_cors/extension.py", line 161, in wrapped_function
    return cors_after_request(app.make_response(f(*args, **kwargs)))
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 1811, in full_dispatch_request
    rv = self.preprocess_request()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/flask/app.py", line 2087, in preprocess_request
    rv = func()
  File "/home/user/Workspace_Py/my_app/app/main.py", line 51, in MiddleWare
    db.connect()
  File "/home/user/Workspace_Py/my_app/lib/python3.5/site-packages/peewee.py", line 2512, in connect
    raise OperationalError('Connection already opened.')
peewee.OperationalError: Connection already opened.
```

this is hapening when i run my app with "gunicorn --workers 3 main:app"
and after crashing (On Peewee Create) , for some minutes it return "peewee.OperationalError: Connection already opened." for all requests ,  Well, you have two tracebacks for this error in your code:

```
ValueError: invalid literal for int() with base 10: 'dsvvgfbgf'
```

The last one looks like the one you referenced in this ticket.

My best guess is that you're leaving some database connections open. Do you open a db connection as part of the app startup? i.e., to create tables or something? Do your views spawn other tasks that use the database and open a connection? You need to be sure to close your connections!

Note that you can also pass `reuse_if_open=True` to the `db.connect()` call, which will not report an error if you forgot to close a connection somewhere:

```python
db.connect(reuse_if_open=True)
```

But this will only mask the problem. thanks for reply , 
no , my "table creation" script is seperated from app and startup ,,
i really just added "connect and close" to "before and after request" , 


my main : 
```
@app.before_request
def MiddleWare():
	db.connect()
        ..........
	pass

@app.after_request
def AfterReq(response):
	if not db.is_closed():
		db.close()
	return response
```

my db_init.py:

```
from peewee import *
db = PostgresqlDatabase('testdb', user='user', password='password',
                           host='localhost', port=5432)

class BaseModel(Model):
    class Meta:
        database = db
```

after these , im importing db and BaseModel to My Models : (I done think this make connection open):

from peewee import *
from db_init import db,BaseModel
```

class Tag(BaseModel):
	class Meta:
		table_name = 'tags_tag'
	tag = CharField(max_length=60,unique=True)
	en_tag = CharField(max_length=60,unique=True)
```
and using my model classes You want `@app.teardown_request`, not `@app.after_request`. I met the same problem. Because I run the code before app create: 
`db.database.create_tables([User, Note])`
and I use `from playhouse.flask_utils import FlaskDB` as  middleware.
when in debug mode. everything is ok.
but when i use gunicorn. it raised:
`    raise OperationalError('Connection already opened.')`

 ```python
with db.database:
    db.database.create_tables([User, Note])
```
im running peewee with flask , 
everything was fine , till i deployed my app with "Gunicorn , 3 workers and gracefull timeout arg"

any idea about using multiple workers with peewee ? 