Question

我有一个Flask视图，它使用SQLAlchemy查询和显示一些博客文章。我正在使用mod_wsgi运行我的应用程序。此视图在我第一次进入页面时起作用，但下次返回500错误。回溯显示错误ProgrammingError: SQLite objects created in a thread can only be used in that same thread.为什么我收到此错误以及如何解决？

views.py

engine = create_engine('sqlite:////var/www/homepage/blog.db')
Base.metadata.bind = engine
DBSession = sessionmaker(bind = engine)
session = DBSession()

@app.route('/blog')
@app.route('/blog.html')
def blog():
    entrys = session.query(Entry).order_by(desc(Entry.timestamp)).all()
    return render_template('blog.html', blog_entrys = entrys)

models.py：

class Entry(Base):
    __tablename__ = 'entry'

    id = Column(Integer, primary_key = True)

    title = Column(String(100), nullable = False)
    body = Column(String, nullable = False)
    timestamp = Column(DateTime, nullable = False)
    featured = Column(Boolean, nullable = False)

    comments = relationship('Comment')

    def is_featured(self):
        return self.featured


class Comment(Base):
    __tablename__ = 'comment'

    id = Column(Integer, primary_key = True)
    entry_id = Column(Integer, ForeignKey('entry.id'))

    text = Column(String(500), nullable = False)
    name = Column(String(80))


engine = create_engine('sqlite:////var/www/homepage/blog.db')
Base.metadata.create_all(engine)

Exception on /blog.html [GET]
Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/flask/app.py", line 861, in wsgi_app
    rv = self.dispatch_request()
  File "/usr/lib/python2.6/dist-packages/flask/app.py", line 696, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/var/www/homepage/webserver.py", line 38, in blog
    entrys = session.query(Entry).order_by(desc(Entry.timestamp)).all()
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/query.py", line 1453, in all
    return list(self)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/query.py", line 1565, in __iter__
    return self._execute_and_instances(context)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/query.py", line 1570, in _execute_and_instances
    mapper=self._mapper_zero_or_none())
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/orm/session.py", line 735, in execute
    clause, params or {})
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/base.py", line 1157, in execute
    params)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/base.py", line 1235, in _execute_clauseelement
    parameters=params
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/base.py", line 1348, in __create_execution_context
    None, None)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/base.py", line 1343, in __create_execution_context
    connection=self, **kwargs)
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/default.py", line 381, in __init__
    self.cursor = self.create_cursor()
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/engine/default.py", line 523, in create_cursor
    return self._connection.connection.cursor()
  File "/usr/lib/python2.6/dist-packages/sqlalchemy/pool.py", line 383, in cursor
    c = self.connection.cursor(*args, **kwargs)
ProgrammingError: (ProgrammingError) SQLite objects created in a thread can only be used in that same thread.The object was created in thread id 140244498364160 and this is thread id 140244523542272 None [{}]

Answer 1

从this SO answer获取提示我搜索了SA文档，发现你可以这样做：

engine = create_engine('sqlite:////var/www/homepage/blog.db?check_same_thread=False')

scoped_session在我的案例中并不合适，因为Flask-SQLAlchemy只接受一个连接字符串参数：

from flask import Flask
from flask_sqlalchemy import SQLAlchemy


class Config(object):
    SQLALCHEMY_DATABASE_URI = 'sqlite:///app.db?check_same_thread=False'


db = SQLAlchemy()


def create_app():
    app.config.from_object(Config)
    app = Flask(__name__)
    db.init_app(app)
    ...

根据sqlite3.connect：

默认情况下，check_same_thread为True，只有创建的主题可能使用连接。如果设置False，则返回的连接可能是跨多个线程共享。 使用多个线程时相同的连接写操作应该由用户序列化避免数据损坏。

Answer 2

如果跨线程共享会话，则SQLAlchemy（在本例中也是SQLite）不起作用。您可能没有显式使用线程，但是mod_wsgi是，并且您已经定义了一个全局session对象。使用scoped_session来处理为每个线程创建唯一会话。

session = scoped_session(sessionmaker(bind=engine))

@app.teardown_request
def remove_session(ex=None):
    session.remove()

@app.route('/')
def example():
    item = session.query(MyModel).filter(...).all()
    ...

最好使用Flask-SQLAlchemy为您处理此事和其他事情。 SQLAlchemy文档建议您使用集成库，而不是自己这样做。

db = SQLAlchemy(app)

@app.route('/')
def example():
    item = db.session.query(MyModel).filter(...).all()
    ...

另请注意，您应该只定义引擎，会话等一次并将其导入其他位置，而不是像当前代码那样在每个文件中重新定义它。

使用Flask中的SQLAlchemy会话引发＆＃34;在线程中创建的SQLite对象只能在同一个线程中使用＆＃34;

2 个答案: