UNIQUE constraint failed in SQLAlchemy

Question

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 the 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 to add 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's record
            paper.handle = v
            self.s.add(paper)
            self.s.commit()
            paper = Paper()

But something goes wrong with authors. After some authors are added to the table, I have (<class 'sqlalchemy.exc.IntegrityError'>, IntegrityError('(IntegrityError) UNIQUE constraint failed: authors.name',), None) error. Meanwhile, the database has only about 50 authors and only one article, while the lines I process contain only about twice as much authors as articles. It means that the script doesn't add them at all.

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

Answer 1

I found a solution that I don't like, but it works. Replace this:

        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)

With this:

        author = self.s.query(Author).filter(Author.name==v).first()
        if author is None:
            author = Author(name=v)

        paper.authors.append(author)