[sqlalchemy] Automatic unique Many-To-Many Relationships

[sjoerd]

Hi,

Is there a way for SQLAlchemy to silently avoid duplicate entries in a 
Many-To-Many Association Table?
My current set-up;
class UserRole(db.Model):
    __tablename__ = 'user_roles'

    id = Column(Integer, primary_key=True)
    rolename = Column(Unicode(80), unique=True)

UserUserRoleTable = Table('user_user_roles', db.Model.metadata,
    Column('user_id', Integer, ForeignKey('users.id')),
    Column('user_role_id', Integer, ForeignKey('user_roles.id')),
    UniqueConstraint('user_id', 'user_role_id', name='uix_1')
)

class User(db.Model):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    roles = relationship("UserRole", secondary=lambda: UserUserRoleTable, 
backref="users")

Code::
    r1 = UserRole('news_add')
    db.session.add(r1)
    r3 = UserRole('news_del')
    db.session.add(r3)

    me = Administrator()
    me.fullname = 'Sjoerd Huisman'
    me.roles.append(r1)
    me.roles.append(r3)
    me.roles.append(r1)
    db.session.add(me)

Without the UniqueConstraint rows are the r1 is added twice. With the 
UniqueConstraint, there is a big error.

Thanks for any thoughts!

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