Question

Sensor有许多Reading个，每个都有一个时间戳的值和类型。我想在Sensor上创建一个属性，列出每种类型的最新值。我知道如何在SQL中获取它，但是如何使用SQLAlchemy构建它？

select * from (
    select * from Readings
    left join Sensors on Readings.SensorID = Sensors.SensorID
    where Readings.SensorID = passed_id order by Timestamp desc
) as tmp group by ValueType;

class Reading(db.Model):
    __tablename__ = 'Readings'
    index = db.Column('id', db.Integer, primary_key=True)
    sensor_id = db.Column('SensorID', db.Integer, db.ForeignKey('Sensors.SensorID'))
    sensor_type = db.Column('SensorType', db.Integer)
    timestamp = db.Column('Timestamp', db.DateTime)
    value_type = db.Column('ValueType', db.Integer)
    value = db.Column('Value', db.Float)

class Sensor(db.Model):
    __tablename__ = 'Sensors'
    index = db.Column('id', db.Integer, primary_key=True)
    id = db.Column('SensorID', db.Integer, unique=True)
    type = db.Column('SensorType', db.Integer)
    location = db.Column('Location', db.String(50))
    readings = db.relationship('Reading',  backref='SensorID', lazy='dynamic')

Answer 1

似乎我终于找到了解决方案。我在Sensor类中添加了以下内容：

rake db:migrate

获取SQLAlchemy中Sensor的每种类型的最新读取值

1 个答案: