对半列Sqlalchemy执行排序

Question

I have a Entity (Table) Name Program and in Program there is a field (Column) Session. Session is a string field as is stored in following way. 2011 - Fall

I only want to sort on year part. Is this possible. I'm using sqlalchemy.

Answer 1

See documentation for SQL and Generic Functions . For postgres it is a substring function which you use to remove the first 7 character from the string. The final query might look like:

from sqlachemy import func
expr = func.substring(Program.session, 8)  # for postgresql
programs = (
    session.query(Program, expr.label("season"))
    .order_by(expr.desc())
    .all()
)

Answer 2

You can sort using desc() or asc() methods. Since you have your string in the format of YEAR - SEASON , this will sort your data like so (assuming descending order):

2019 - Spring
2019 - Fall
2018 - Spring
2018 - Fall
2017 - Spring
2017 - Fall
2016 - Spring
2016 - Fall
2015 - Spring
2015 - Fall
2014 - Spring
2014 - Fall
...

---

This can be seen with the following example:

from sqlalchemy import Column, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

class Program(Base):
    __tablename__ = 'program'
    session = Column(String(250), primary_key=True)

engine = create_engine('sqlite:///program.db')
Base.metadata.create_all(engine)
Base.metadata.bind = engine
DBSession = sessionmaker(bind=engine)
session = DBSession()

for x in range(2000, 2020):
    session.add(Program(session="{} - Fall".format(x)))
    session.add(Program(session="{} - Spring".format(x)))

session.commit()
programs = session.query(Program).order_by(Program.session.desc()).all()

for x in programs:
    print x.session

---

This creates a <Year> - Fall and <Year> - Spring entry for each year between 2000 and 2019. It then runs a simple query asking for all of this data back in descending order. The first several lines are at the top of this answer.