Checking db-models on uniqueness. INSERT OR IGNORE by means of Flask-SQLAlchemy

Question

I have a question on how to add new but unique items to the database.

app = Flask(__name__, template_folder='templates')
app.config.from_object('config')
db = SQLAlchemy(app)

Classes look like this many-to-many implementation:

connections = db.Table('connections', db.metadata,
    db.Column('book_id', db.Integer, db.ForeignKey('books.id')),
    db.Column('author_id', db.Integer, db.ForeignKey('authors.id'))
)

class Author(db.Model):
    __tablename__ = 'authors'
    __searchable__ = ['a_name']
    __table_args__ = {'sqlite_autoincrement': True,}

    id = db.Column(db.Integer, primary_key=True)
    a_name = db.Column(db.String(80), unique=True)

    def __repr__(self):
        return unicode(self.a_name)


class Book(db.Model):
    __tablename__ = 'books'
    __searchable__ = ['b_name']
    __table_args__ = {'sqlite_autoincrement': True,}

    id = db.Column(db.Integer, primary_key=True)
    b_name = db.Column(db.String(80), unique=True)
    authors = db.relationship('Author', secondary=lambda: connections,
                                                    backref=db.backref('books'))

    def __repr__(self):
        return unicode(self.b_name)

I want to add only unique db items. If I will write this code:

author = Author(a_name = "Author1")
book = Book(b_name = "Book1")
author.books.append(book)
db.session.add(author)
db.session.add(book)
db.session.commit()

And we already had author with a_name "Author1" in our database there will be the error exeption.

IntegrityError: (IntegrityError) column a_name is not unique u'INSERT INTO authors (a_name) VALUES (?)' (u'\u0410\u0432\u0442\u043e\u04402',)

Do I need to check the uniqueness of this insershions and how? Or there is another best solution?

Answer 1

the ORM doesn't have support for the "MERGE" approach (e.g. "INSERT OR REPLACE" and all that) and there's a lot of complications with those as well. I use the unique object recipe or some variant thereof. When I'm doing a large data merge, I'll frequently load up the entire set of objects to be dealt with for some particular chunk into a dictionary ahead of time, so there's just one SELECT statement to load them, then just check into that dictionary as I proceed.

Answer 2

Actually I found not easy going solution and ugly one :), but it works. Of course, when I will use a big database I would use this unique object recipe which @zzzeek introduced.

new_author = Author(a_name = request.form['author'])
new_book = Book(b_name = request.form['book'])
author_in_db = Author.query.filter_by(a_name=unicode(new_author)).first()
book_in_db = Book.query.filter_by(b_name=unicode(new_book)).first()
# both author and book are new and unique
if unicode(new_author) != unicode(author_in_db) and \
                               unicode(new_book) != unicode(book_in_db):
    new_author.books.append(new_book)
    db.session.add(new_author)
    db.session.add(new_book)
    db.session.commit()
# just book is not unique
elif unicode(new_author) != unicode(author_in_db):
    new_author.books.append(book_in_db)
    db.session.add(new_author)
    db.session.commit()
# just author is not unique
elif unicode(new_book) != unicode(book_in_db):
    author_in_db.books.append(new_book)
    db.session.add(new_book)
    db.session.commit()

Answer 3

class User(Base):
  __tablename__ = 'users'

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

insert_command = User.__table__.insert(
    prefixes=['OR IGNORE'],
    values=dict(name='MyName', fullname='Slim Shady', password='******')
)
session.execute(insert_command)
session.commit() 

Replace works as well

prefixes=['OR REPLACE']