Flask 应用程序中的 Sqlalchemy 意外级联删除

当我从我的数据库中删除一个项目时，我得到了一个奇怪的级联删除（我没有配置）。删除项对应的分类条目被删除。我如何阻止这种情况发生？

这是我的数据库设置：

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy import create_engine


Base = declarative_base()


class User(Base):
    """Object holding information about individual registered users"""

    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)
    email = Column(String(250), nullable=False)
    picture = Column(String(250))


class Category(Base):
    """Object holding information about item categories"""

    __tablename__ = 'category'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)


class Item(Base):
    """Object holding information about individual catalog items"""

    __tablename__ = 'item'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)
    description = Column(String(5000), nullable=False)
    category_id = Column(Integer, ForeignKey('category.id'))
    category = relationship("Category")
    user_id = Column(Integer, ForeignKey('user.id'))
    user = relationship("User")


engine = create_engine('postgresql+psycopg2://********:********'
                       + '/itemcatalog.db', echo=False)
Base.metadata.create_all(engine)

这是触发删除的代码：

@app.route('/item/<int:item_id>/delete/', methods=['GET', 'POST'])

def delete_item(item_id):
    item = session.query(Item).filter_by(id=item_id).one()

    if request.method == 'POST':
        session.delete(item)
        session.commit()
        flash('Item Successfully Deleted')
        return redirect(url_for('show_catalog'))
    else:
        return render_template('deleteitem.html', item=item,
                               username=login_session['username'],
                               picture=login_session['picture'])

预期的行为是仅删除由其“id”指定的项目，并且不会更改其他表。

我得到的实际结果是该item被删除，该item对应的category表中的条目也被删除了。

更新：

我正在采取的步骤是：

1. 在数据库中创建一个新项目
2. 删除该项目

显示这一点的日志（和删除）如下。

添加新项目：

2017-12-06 19:32:16 UTC [5383-22] [email protected] LOG:  statement: SELECT category.id AS category_id, category.name AS category_name 
    FROM category
2017-12-06 19:32:16 UTC [5383-23] [email protected] LOG:  statement: INSERT INTO item (name, description, category_id, user_id) VALUES ('Coffee', 'tasty beverage', 3, 2) RETURNING item.id
2017-12-06 19:32:16 UTC [5383-24] [email protected] LOG:  statement: COMMIT
2017-12-06 19:32:16 UTC [5383-25] [email protected] LOG:  statement: BEGIN
2017-12-06 19:32:16 UTC [5383-26] [email protected] LOG:  statement: SELECT category.id AS category_id, category.name AS category_name 
    FROM category ORDER BY category.name ASC
2017-12-06 19:32:16 UTC [5383-27] [email protected] LOG:  statement: SELECT item.id AS item_id, item.name AS item_name, item.description AS item_description, item.category_id AS item_category_id, item.user_id AS item_user_id 
    FROM item ORDER BY item.name ASC

删除新项目：

2017-12-06 19:33:14 UTC [5383-30] [email protected] LOG:  statement: SELECT item.id AS item_id, item.name AS item_name, item.description AS item_description, item.category_id AS item_category_id, item.user_id AS item_user_id 
    FROM item 
    WHERE item.id = 7
2017-12-06 19:33:14 UTC [5383-31] [email protected] LOG:  statement: SELECT category.id AS category_id, category.name AS category_name 
    FROM category 
    WHERE category.id = 3
2017-12-06 19:33:14 UTC [5383-32] [email protected] LOG:  statement: SELECT item.id AS item_id, item.name AS item_name, item.description AS item_description, item.category_id AS item_category_id, item.user_id AS item_user_id 
    FROM item 
    WHERE 3 = item.category_id
2017-12-06 19:33:14 UTC [5383-33] [email protected] LOG:  statement: UPDATE item SET category_id=NULL WHERE item.id = 5
2017-12-06 19:33:14 UTC [5383-34] [email protected] LOG:  statement: UPDATE item SET category_id=NULL WHERE item.id = 6
2017-12-06 19:33:14 UTC [5383-35] [email protected] LOG:  statement: DELETE FROM item WHERE item.id = 7
2017-12-06 19:33:14 UTC [5383-36] [email protected] LOG:  statement: DELETE FROM category WHERE category.id = 3
2017-12-06 19:33:14 UTC [5383-37] [email protected] LOG:  statement: COMMIT
2017-12-06 19:33:14 UTC [5383-38] [email protected] LOG:  statement: BEGIN
2017-12-06 19:33:14 UTC [5383-39] [email protected] LOG:  statement: SELECT category.id AS category_id, category.name AS category_name 
    FROM category ORDER BY category.name ASC
2017-12-06 19:33:14 UTC [5383-40] [email protected] LOG:  statement: SELECT item.id AS item_id, item.name AS item_name, item.description AS item_description, item.category_id AS item_category_id, item.user_id AS item_user_id 
    FROM item ORDER BY item.name ASC

从日志中我可以看到它的行为不像我想要的那样，但我不知道要改变什么来阻止这种行为。