Flask + SQLalchemy计算字段过滤器

Question

目前，我有一个名为calc_totalPrice的混合属性的类。 在此混合属性中，我尝试根据与另一个表（通过中间表）相关的多个项目来计算某事物的总价。 这是课程：

class Recipe(db.Model):

    recipeID = Column(Integer, primary_key=True)
    userID = Column(ForeignKey('user.userID'), nullable=False)
    name = Column(String(35), nullable=False)
    description = Column(String(140), nullable=False)

    User = db.relationship('User')

    @hybrid_property
    def calc_totalPrice(self):
        calculatedPrice = func.sum(Ingredient.price).label('price')
        recipeIngredientJoin = Recipe.query.join(IngredientsToRecipe,Recipe.recipeID == IngredientsToRecipe.recipeID).join(Ingredient,IngredientsToRecipe.siin == Ingredient.siin).add_columns(calculatedPrice).group_by(Recipe.recipeID).filter(Recipe.recipeID == self.recipeID).first()
        print(calculatedPrice)
        return calculatedPrice

我尝试运行的方法如下所示：

@app.route('/api/tasks/getrecipetypeinbudget/<float:budget>', methods=['GET'])
def GetRecipeTypeInBudget(budget):
    recipeIngredients = Recipe.query.filter(Recipe.calc_totalPrice <= budget)
    serialiser = RecipeSerializer(many = True)
    result = serialiser.dump(recipeIngredients)

    return jsonify({'Recipe' : result})

不幸的是，我收到以下错误：

 * Running on http://0.0.0.0:53827/ (Press CTRL+C to quit)
127.0.0.1 - - [10/Apr/2018 00:27:11] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:11] "GET /static/content/bootstrap.min.css HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:12] "GET /static/scripts/modernizr-2.6.2.js HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:12] "GET /static/scripts/respond.js HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:12] "GET /static/content/site.css HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:12] "GET /static/scripts/bootstrap.js HTTP/1.1" 200 -
127.0.0.1 - - [10/Apr/2018 00:27:12] "GET /static/scripts/jquery-1.10.2.js HTTP/1.1" 200 -
sum(ingredient.price)
[2018-04-10 00:27:27,382] ERROR in app: Exception on /api/tasks/getrecipetypeinbudget/5.0 [GET]
Traceback (most recent call last):
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 1193, in _execute_context
    context)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
psycopg2.ProgrammingError: aggregate functions are not allowed in WHERE
LINE 3: WHERE sum(ingredient.price) <= 5.0
              ^

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\_compat.py", line 33, in reraise
    raise value
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\flask\app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\app\views.py", line 46, in GetRecipeTypeInBudget
    result = serialiser.dump(recipeIngredients)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\marshmallow\schema.py", line 481, in dump
    obj = list(obj)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\orm\query.py", line 2889, in __iter__
    return self._execute_and_instances(context)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\orm\query.py", line 2912, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\sql\elements.py", line 269, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 1060, in _execute_clauseelement
    compiled_sql, distilled_params
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 1200, in _execute_context
    context)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\util\compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\util\compat.py", line 186, in reraise
    raise value.with_traceback(tb)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\base.py", line 1193, in _execute_context
    context)
  File "C:\Users\Samuel.endeva\Google Drive\Projects\Sal\SalApp\SalApp\FlaskWebAPI\env\lib\site-packages\sqlalchemy\engine\default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) aggregate functions are not allowed in WHERE
LINE 3: WHERE sum(ingredient.price) <= 5.0
              ^
 [SQL: 'SELECT recipe."recipeID" AS "recipe_recipeID", recipe."userID" AS "recipe_userID", recipe.name AS recipe_name, recipe.description AS recipe_description \nFROM recipe, ingredient \nWHERE sum(ingredient.price) <= %(param_1)s'] [parameters: {'param_1': 5.0}] (Background on this error at: http://sqlalche.me/e/f405)
127.0.0.1 - - [10/Apr/2018 00:27:27] "GET /api/tasks/getrecipetypeinbudget/5.0 HTTP/1.1" 500 -

我今天才开始使用Flask，虽然我可以大致了解我要去哪里的问题，但我不知道如何解决。 有什么想法或选择吗？

更新：我取得了一些成功。 我将混合属性更改为：

@hybrid_property
def calc_totalPrice(self):
    calculatedPrice = func.sum(Ingredient.price).label('price')
    recipeIngredientJoin = Recipe.query.join(IngredientsToRecipe,Recipe.recipeID == IngredientsToRecipe.recipeID).join(Ingredient,IngredientsToRecipe.siin == Ingredient.siin).add_columns(calculatedPrice).group_by(Recipe.recipeID).filter(Recipe.recipeID == self.recipeID).first()
    return recipeIngredientJoin.price

问题是，这会将所有价格列都设置为第一个值。 使用.first到.all返回一个列表，这很不方便。 有任何提示或想法吗？

Answer 1

您首次尝试混合属性时，有点太混合了。 它执行查询，但返回SQL表达式对象，即聚合函数表达式，该对象在WHERE子句中不可用。 您应该将两者分开：

class Recipe(db.Model):

    # The relationship makes defining the hybrid property easier on the
    # Python side.
    ingredients = db.relationship('Ingredient', secondary='ingredients_to_recipe')

    @hybrid_property
    def calc_totalPrice(self):
        # Take a simple sum over ingredient prices, using the
        # relationship. This is meant to take place in Python.
        return sum(i.price for i in self.ingredients)

    @calc_totalPrice.expression
    def calc_totalPrice(cls):
        # Create a correlated subquery, usable in WHERE clauses, SELECT lists etc.
        # In other words, this is for SQL.
        return db.session.query(func.sum(Ingredient.price)).\
            join(IngredientsToRecipe).\
            filter_by(recipeID=cls.recipeID).\
            label('price')

这样的混合属性可方便地检查单个或几个Recipe实例的总价。 在给定合适的索引的情况下，即使对于较大的查询，相关的标recipeID查询也可能表现良好，例如，应使按recipeID查找IngredientsToRecipe变得容易。 但这不是您唯一的选择。 如果您需要更具体的查询，请务必使用此类查询。 SQLAlchemy的美丽之处在于它允许与ORM一起使用关系数据库的功能：

price = func.sum(Ingredient.price).label('price')

# Note that using `User` as the name of an attribute is a bit confusing.
# Doing `group_by(Recipe.recipeID)` instead of `group_by(Recipe)` is ok,
# at least in databases that recognize that the other non-aggregate columns
# have a functional dependency to the grouping column, or in other words
# `Recipe.recipeID` defines the rest. Be wary about databases that allow
# non-aggregate columns in a group even without such a dependency. You
# will probably get indeterminate results.
cheap_recipes = db.session.query(Recipe, price).\
    filter_by(User=some_user).\
    join(Recipe.ingredients).\
    group_by(Recipe.recipeID).\
    having(price <= 5.0).\
    all()