SQLAlchemy混合属性表达式日期时间转换

我正在用python 2.7使用flask-sqlalchemy。SQLAlchemy混合属性表达式日期时间转换

我试图创建一个属性，我可以在查询中使用来排序。我已经了解到我应该使用hybrid_property来实现这一点，但我无法在表达式中获得timedelta对象。

我想执行的查询是一样的东西Article.query.filter(Article.language == language).order_by(Article.popularity.desc())

与

from math import exp 
from datetime import datetime, timedelta 
from sqlalchemy.ext.hybrid import hybrid_property 

class Article(db.Model): 
    __tablename__ = 'articles' 
    article_id = db.Column(db.Integer, primary_key=True) 
    # ... 
    language = db.Column(db.String(64), default='en') 
    date_time = db.Column(db.DateTime, default=datetime.utcnow) 
    readers_frac = db.Column(db.Float, default=0.) 

    @hybrid_property 
    def popularity(self): 
     # compute the popularity based on the readers_frac and the time in minutes since the article was published 
     min_since_published = (datetime.utcnow() - self.date_time).total_seconds()/60. 
     popularity_decay = exp(-(min_since_published**2/(2.*1832.4**2))) 
     return self.readers_frac*popularity_decay 

    @popularity.expression 
    def popularity(cls): 
     # compute the popularity based on the readers_frac and the time in minutes since the article was published 
     min_since_published = (datetime.utcnow() - cls.date_time).seconds/60. 
     popularity_decay = exp(-(min_since_published**2/(2.*1832.4**2))) 
     return cls.readers_frac*popularity_decay

但是试图计算min_since_published时，我发现了错误AttributeError: Neither 'BinaryExpression' object nor 'Comparator' object has an attribute 'seconds'。任何提示？谢谢！！

UPDATE

我固定它通过改变一切sqlalchemy.func表达式，即它的工作原理（与MySQL后端）与

import sqlalchemy as sa 
@popularity.expression 
def popularity(cls): 
    # compute the popularity based on the readers_frac and the time in minutes since the article was published 
    min_since_published = sa.func.timestampdiff(sa.text('MINUTE'), cls.date_time, sa.func.utc_timestamp()) 
    popularity_decay = sa.func.exp(-(sa.func.pow(min_since_published, 2)/6730009.)) 
    return cls.readers_frac*popularity_decay

来源

2016-07-06 cod3licious

您使用哪个数据库？（日期/时间功能在它们之间变化很大） – RazerM

这是因为'datetime.utcnow（） - cls.date_time'是一个* SQL表达式*，因此不像'datetime.utcnow（） - self那样具有'.seconds'。 date_time'，这是'datetime'。您需要在您的RDBMS中找到相应的功能，以提供相应的功能。（与'exp'一样的东西。） – univerio

现在我在后台使用SQLite，但后来它将成为mysql。我希望sqlalchemy抽象这不会有问题... – cod3licious

我学到它实际上取决于后端数据库您'正在使用它以及它提供的功能。例如，sqlite不会工作，因为数据库本身不具备回答查询的功能，但mysql可以。然后，它只是看看函数如何看起来像一个原始的mysql查询语句，并将其翻译成sqlalchemy.func表达式。这工作：

import sqlalchemy as sa 
@popularity.expression 
def popularity(cls): 
    # compute the popularity based on the readers_frac and the time in minutes since the article was published 
    min_since_published = sa.func.timestampdiff(sa.text('MINUTE'), cls.date_time, sa.func.utc_timestamp()) 
    popularity_decay = sa.func.exp(-(sa.func.pow(min_since_published, 2)/6730009.)) 
    return cls.readers_frac*popularity_decay

来源

2016-08-05 17:10:06 cod3licious

SQLAlchemy混合属性表达式日期时间转换

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