从烧瓶视图中分离芹菜任务分离SQLAlchemy实例（DetachedInstanceError）

Question

I am using SQLAlchemy models (derived from sqlalchemy.ext.declarative.declarative_base) together with Flask-SQLAlchemy

When I try to run any celery task (just empty)

@celery.task()
def empty_task():
    pass

in common flask view

@blueprint.route(...)
def view():
   image = Image(...)
   db.session.add(image)
   db.session.flush()

   #this cause later error
   empty_task()

   #now accessing attributes ends with DetachedInstanceError
   return jsonify({'name': image.name, ...}

i get

DetachedInstanceError: Instance <Image at 0x7f6d67e37b50> is not bound to a Session; attribute refresh operation cannot proceed

when I trying access model after task push. Without task it works fine. How to fix it?

update: celery use this task base:

TaskBase = celery.Task
class ContextTask(TaskBase):
    abstract = True

    def __call__(self, *args, **kwargs):
        with app.app_context():
            try:
                return TaskBase.__call__(self, *args, **kwargs)
            except Exception:
                sentry.captureException()
                raise

celery.Task = ContextTask

Answer 1

ah my mistake in running task. it should be empty_task.apply_async()

calling it directly it creates new app context with new session causing closing old one.

Answer 2

Today I had the same issue while I was running my nose tests.

DetachedInstanceError: Instance <EdTests at 0x1071c4790> is not bound to a Session; attribute refresh operation cannot proceed

I am using Celery and Flask SQLAlchemy. Issue was caused when I changed in testing settings:

CELERY_ALWAYS_EAGER = True

I had found that when running celery tasks synchronously db session is closed at the end of the task.

I solved my issue following Celery's documentation user guide. Celery recommends not to enable eager testing of tasks.