title: SQLAlchemy: add a relationship using id instead of object?
body: <p>Is it possible to add to a SQLAlchemy relationship using ids rather than objects?</p>
  
  <p>For example, consider two declarative SQLAlchemy classes, Review and Artist, with a relationship between them:</p>
  
  <pre><code>class Review(Base):
      artist_id = Column(Integer, ForeignKey('artist.id'))
      artist = relationship(Artist, backref=backref('reviews', order_by=id))
      # etc.
  
  class Artist(Base):
      # etc.
  </code></pre>
  
  <p>With a list of review ids to add to an artist, I seem to need to look up the artist from the id, then add the artist object to the review, like this:</p>
  
  <pre><code>for review_id in review_ids:
      review = session.query(Review).filter(Review.id==review_id).first()
      artist.reviews.append(review)
  </code></pre>
  
  <p>I'm sure it would be more efficient to skip the lookup and just add the ids, but is this possible?</p>