gistfile1.py

class Entry(Base):
    __tablename__ = 'entries'
    id = Column(Integer, primary_key=True)
    text = Column(String(200))
    created_at = Column(DateTime, default=datetime.now, nullable=False)

    comments = relationship('Comment', backref="entry",
                    primaryjoin="Entry.id==Comment.entry_id", # <- ここ
                    foreign_keys="Comment.entry_id")          # <- ここ

class Comment(Base):
    __tablename__ = 'comments'
    id = Column(Integer, primary_key=True)
    entry_id = Column(Integer, nullable=False)                 # <= ForeignKeyを外す
    text = Column(String(200))
    created_at = Column(DateTime, default=datetime.now, nullable=False)