使用SqlAlchemy的Select语句

Question

Yes, very basic question. I've successfully created my db using declarative_base, and can perform inserts into the db too. I just have a few questions about SqlAlchemy sql statements.

I've create a table called Location.
A few issues/questions (see code below):

1. For statement, "print row", I have to specify each column name that I want to have output. ie "print row.name, row.lat, etc" Why? (Otherwise the print statement outputs "<classname.Location at <...>>"

2. Also, what is the preferred way to interact with a db and perform queries (select, insert, update, etc.)- there seem to be a bunch of options: using sqlalchemy.orm.select for example, or engine.text (<sql query>).execute().fetchall() , or even conn.execute(<select>). Options are great, but right now they're all just confusing me.

Thanks so much for the tips!

Here's my code:

from sqlalchemy import create_engine
from sqlalchemy.sql import select
from location_db_setup import *

db_path = "sqlite:////volumes/users/shared/programming/python/web/map.db"
engine = create_engine(db_path, echo= True)
Session = sessionmaker(bind= engine)
session = Session()

session.query(Location).fetchall()
for row in locations:
        print row

Answer 1

1. You code in sample is incomplete and has errors. So it's impossible to say for sure what is Location here. I assume it's a mapped class, so you are requesting a list of all Location objects , not rows . When you print an object you get its string representation. String representation of objects can be changed by defining custom __str__ method.
2. Although ORM is the most important part of SQLAlchemy, it's not the only. It also expose a lot of functionality not related to ORM directly. When you work with objects the preferred way to create queries are corresponding session method. But sometimes you need selectable objects not bound to particular session (they are not executed directly, but are used in expressions passed to session methods). That's why there are functions in sqlalchemy.orm package.

Answer 2

The preferred way to interact with a db when using an ORM is not to use queries but to use objects that correspond to the tables you are manipulating, typically in conjunction with the session object. SELECT queries become get() or find() calls in some ORMs, query() calls in others. INSERT becomes creating a new object of the type you want (and maybe explicitly adding it, eg session.add() in sqlalchemy). UPDATE becomes editing such an object, and DELETE becomes deleting an object (eg. session.delete() ). The ORM is meant to handle the hard work of translating these operations into SQL for you.

Have you read the tutorial ?

Answer 3

Denis and Kylotan gave you good answers. I'm just gonna focus on point 2.

Sometimes depends on your taste. There are times when you need database specific features that an ORM can't do, that's a case when you should use Session(<sql here>).execute() or conn.execute(<sql here>) . Another case is when you have a very complex query which is beyond you and you don't find a suitable ORM expression.

Usually, using ORM features like select([...]).where(... or Session.query(<Model here>).filter(... (declarative base) are enough. Almost every sql query has an ORM equivalent.