SQL Alchemy PostgreSQL从ORM模式创建表时出错没有唯一键约束

我有一个作者书的简单模型,但我找不到一种方法来将firstName和lastName作为复合键,并在关系中使用它。有什么想法吗?

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

Base = declarative_base()
engine = create_engine('mssql://user:pass@library')
engine.echo = True
session = sessionmaker(engine)()

class Author(Base):
    __tablename__ = 'authors'
    firstName = Column(String(20), primary_key=True)
    lastName = Column(String(20), primary_key=True)
    books = relationship('Book', backref='author')

class Book(Base):
    __tablename__ = 'books'
    title = Column(String(20), primary_key=True)
    author_firstName = Column(String(20), ForeignKey('authors.firstName'))
    author_lastName = Column(String(20), ForeignKey('authors.lastName'))