title: Relations on composite keys using sqlalchemy
body: <p>I have this simple model of Author - Books and can't find a way to make firstName and lastName a composite key and use it in relation. Any ideas?</p>
  
  <pre><code>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'))            
  </code></pre>