接著前面的例子說，我們定義了book_table和author_table，接下來：
?1?class?Book(object):
?2?????def?__init__(self,?title):
?3?????????self.title?=?title
?4?????def?__repr__(self):
?5?????????return?"<Book('%s')>"?%?self.title
?6?
?7?class?Author(object):
?8?????def?__init__(self,?name):
?9?????????self.name?=?name
10?????def?__repr__(self):
11?????????return?"<Author('%s')>"?%?self.name
這里我們定義兩個類，繼承自object，類似JavaBeans或者POJO，這里的__init__方法和__repr__方法不是必須的，只是為了創建對象和輸出對象內容比較方便。然后就可以用SQLAlchemy的mapper和sessionmaker來建立映射關系并處理持久和查詢等操作：
?1?from?sqlalchemy.orm?import?mapper,sessionmaker
?2?
?3?mapper(Book,?book_table)
?4?mapper(Author,?author_table)
?5?
?6?Session?=?sessionmaker(bind=engine)
?7?session?=?Session()
?8?
?9?gia?=?Book(u'Groovy?in?Action')
10?ag?=?Author(u'Andrew?Glover')
11?
12?session.add(gia)
13?session.add(ag)
14?session.add_all([Book('Hibernate?in?Action'),?Author('Gavin?King')])
15?s_gia?=?session.query(Book).filter_by(title=u'Groovy?in?Action').first()
16?s_gia.title?=u'Groovy?in?Action?Updated'
17?
18?print?"[DIRTY]",?session.dirty
19
20?session.commit() # or session.rollback()
如果你用過Hibernate，那么這些代碼對你來說，理解起來應該沒有任何難度。
假如我告訴你，每次都要像這樣先定義Table(schema)，再定義class，然后用mapper建立對照，是不是有點那啥？SQLAlchemy的開發者們也意識到這一點，所以從0.5開始，SQLAlchemy可以通過sqlalchemy.ext.declarative支持我們實現更緊湊的model/schema定義：
?1?from?sqlalchemy.schema?import?Table,?Column,?ForeignKey,?Sequence
?2?from?sqlalchemy.types?import?*
?3?from?sqlalchemy.orm?import?relation
?4?from?sqlalchemy.ext.declarative?import?declarative_base
?5?
?6?Base?=?declarative_base()
?7?metadata?=?Base.metadata
?8?
?9?bookauthor_table?=?Table('bookauthor',?metadata,
10?????Column('book_id',?Integer,?ForeignKey('book.id'),?nullable=False),
11?????Column('author_id',?Integer,?ForeignKey('author.id'),?nullable=False),
12?)
13?
14?class?Book(Base):
15?????__tablename__?=?'book'
16?????id?=?Column(Integer,?Sequence('seq_pk'),?primary_key=True)
17?????title?=?Column(Unicode(255),?nullable=False)
18?????authors?=?relation('Author',?secondary=bookauthor_table)
19?
20?
21?class?Author(Base):
22?????__tablename__?=?'author'
23?????id?=?Column(Integer,?Sequence('seq_pk'),?primary_key=True)
24?????name?=?Column(Unicode(255),?nullable=False)
25?????books?=?relation('Book',?secondary=bookauthor_table)
這里我們用到了many-to-many關系，其他的常見用法還包括many-to-one、one-to-many、JOIN、子查詢、EXISTS、Lazy/Eager Load、Cascade (all/delete/delete-orphan)等等，大家可以根據需要查閱官方文檔。