编辑“︁SQLAlchemy”︁（章节）

== 示例 ==
下述示例描述了电影同它们的导演之间的多对一[[实体联系模型|联系]]。示例中说明了怎样从用户定义的Python[[类 (计算机科学)|类]]创建对应的数据库表格，怎样从[[实体联系模型|联系]]的任何一方创建有关联的[[实例 (计算机科学)|实例]]，最终怎样查询数据，演示了为延迟加载和预先加载二者自动生成的[[SQL]]查询。

=== 架构定义 ===
创建两个Python类以及在[[DBMS]]数据库[[Schema (数据库)|架构]]中对应的表格：

<syntaxhighlight lang="Python">
from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relation, sessionmaker

Base = declarative_base()
 
class Movie(Base):
    __tablename__ = 'movies'
 
    id = Column(Integer, primary_key=True)
    title = Column(String(255), nullable=False)
    year = Column(Integer)
    directed_by = Column(Integer, ForeignKey('directors.id'))
 
    director = relation("Director", backref='movies', lazy=False)
 
    def __init__(self, title=None, year=None):
        self.title = title
        self.year = year
    def __repr__(self):
        return "Movie(%r, %r, %r)" % (self.title, self.year, self.director)
 
class Director(Base):
    __tablename__ = 'directors'
 
    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False, unique=True)
 
    def __init__(self, name=None):
        self.name = name
 
    def __repr__(self):
        return "Director(%r)" % (self.name)
 
engine = create_engine('dbms://user:pwd@host/dbname')
Base.metadata.create_all(engine)

</syntaxhighlight>

=== 插入数据 ===
电影与导演[[实体联系模型|联系]]可以通过任何一方[[实体联系模型|实体]]插入：
<syntaxhighlight lang="Python">
Session = sessionmaker(bind=engine)
session = Session()

m1 = Movie("Star Trek", 2009)
m1.director = Director("JJ Abrams")

d2 = Director("George Lucas")
d2.movies = [Movie("Star Wars", 1977), Movie("THX 1138", 1971)]

try:
    session.add(m1)
    session.add(d2)
    session.commit()
except:
    session.rollback()
</syntaxhighlight>

=== 查询 ===
<syntaxhighlight lang="Python">
alldata = session.query(Movie).all()
for somedata in alldata:
    print somedata
</syntaxhighlight>

SQLAlchemy将向DBMS发起如下查询（忽略别名）：
<syntaxhighlight lang="SQL">
SELECT movies.id, movies.title, movies.year, movies.directed_by, directors.id, directors.name 
FROM movies LEFT OUTER JOIN directors ON directors.id = movies.directed_by
</syntaxhighlight>

输出结果：
<syntaxhighlight lang="python">
Movie('Star Trek', 2009L, Director('JJ Abrams'))
Movie('Star Wars', 1977L, Director('George Lucas'))
Movie('THX 1138', 1971L, Director('George Lucas'))
</syntaxhighlight>

假如在架构定义时设置<code>lazy=True</code>（默认值），SQLAlchemy将首先发起一个查询来获得一个电影列表，并在必要时（延迟）对每个导演发起查询来获得对应导演的名字：
<syntaxhighlight lang="SQL">
SELECT movies.id, movies.title, movies.year, movies.directed_by 
FROM movies

SELECT directors.id, directors.name
FROM directors 
WHERE directors.id = %s
</syntaxhighlight>