Conditional UNIQUE constraint in SQLAlchemy

I have two simple classes for SQLAlchemy:

papers2authors_table = Table('papers2authors', Base.metadata,
    Column('paper_id', Integer, ForeignKey('papers.id')),
    Column('author_id', Integer, ForeignKey('authors.id'))
)

class Paper(Base):
    __tablename__ = "papers"

    id = Column(Integer, primary_key=True)
    title = Column(String)
    handle = Column(String)

    authors = relationship("Author",
                    secondary="papers2authors",
                    backref="papers")

class Author(Base):
    __tablename__ = "authors"

    id = Column(Integer, primary_key=True)
    name = Column(String, unique=True)
    code = Column(String, unique=True)

      

        
        
        
      

    

Then I run init elsewhere:

    engine = create_engine('sqlite:///' + REPECI_DB, echo=True)
    Base.metadata.create_all(engine)
    Session = sessionmaker(bind=engine)
    session = Session()
    self.s = session

      

        
        
        
      

    

And try adding items to papers

and authors

:

    paper = Paper()
    for line in lines: # the data is a sequence of lines "key: value" with few papers per file
        br = line.find(':')
        k = line[:br]
        v = line[br+1:].strip()

        if k == "Title":
            paper.title = v
        elif k == "Year":
            paper.year = v
        elif k == "Author-Name":
            try:
                self.s.begin_nested()
                author = Author(name=v)
            except IntegrityError:
                print("Duplicate author")
                self.s.rollback()
                author = self.s.query(Author).filter(Author.name==v).first()
            else:
                self.s.commit()
            paper.authors.append(author)
        elif k == "Handle": # this appears in the end of a paper record
            paper.handle = v
            self.s.add(paper)
            self.s.commit()
            paper = Paper()

      

        
        
        
      

    

But something is wrong with the authors. After some authors are added to the table, I have an error (<class 'sqlalchemy.exc.IntegrityError'>, IntegrityError('(IntegrityError) UNIQUE constraint failed: authors.name',), None)

. Meanwhile, the database has only about 50 authors and only one article, and the rows I process contain about twice as many authors as articles. This means the script doesn't add them at all.

I tried to rewrite the code as recommended here , but the error still appears.