sqlalchemy一对多关系加入？

我正在尝试这样的简单联接查询,

SELECT food._id, food.food_name, food_categories.food_categories FROM food JOIN food_categories ON food.food_category_id = food_categories._id

但仍会收到错误消息.这是我的班级设置方式.

class Food_Categories(db.Model):
    __tablename__ = 'food_categories'
    _id = db.Column(db.Integer, primary_key=True)
    food_categories = db.Column(db.String(30))

class Food(db.Model):
    __tablename__ = 'food'
    _id = db.Column(db.Integer, primary_key=True)
    food_name = db.Column(db.String(40))
    food_category_id = db.Column(db.Integer, ForeignKey(Food_Categories._id))
    food_category = relationship("Food_Categories")

我的查询功能如下所示.

@app.route('/foodlist')
def foodlist():
    if request.method == 'GET':
        results = Food.query.join(Food_Categories.food_categories).all()

    json_results = []
    for result in results:
        d = {'_id': result._id,
         'food': result.food_name,
         'food_category': result.food_categories}
    json_results.append(d)

    return jsonify(user=json_results)

我正在使用Flask.当我呼叫路线时,出现此错误.

AttributeError: 'ColumnProperty' object has no attribute 'mapper'

我本质上想要这样：

|      id       |    food_name    |    food_category    |

并将food_category_id列替换为位于其他表中的食品类别的实际名称.

我的表/关系设置正确吗？我的查询设置正确吗？

解决方法:

您的表和关系已正确设置.您的查询需要更改.

错误的原因是您尝试对列(Food_Categories.food_categories)而非表(或映射的模型对象)执行联接.从技术上讲,您应该使用以下查询替换您的查询以修复错误：

results = Food.query.join(Food_Categories).all()

这将解决错误,但不会生成您想要的SQL语句,因为即使存在联接,它也只会作为结果返回Food的实例.

为了构建一个查询,该查询将准确生成您要记住的SQL语句：

results = (db.session.query(Food._id, Food.food_name,
        Food_Categories.food_categories,)
    .join(Food_Categories)).all()
for x in results:
    # print(x)
    print(x._id, x.food_name, x.food_categories)

请注意,在这种情况下,结果不是Food的实例,而是具有3列值的元组.

标签：sqlalchemy,flask,flask-sqlalchemy,python
来源： https://codeday.me/bug/20191121/2055021.html