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I am dealing with a many-to-many relationship with sqlalchemy. My question is how to avoid adding duplicate pair values in a many-to-many relational table. To make things clearer, I will use the example from the official SQLAlchemy documentation. Base = declarative_base() Parents2children = Table('parents2children', Base.metadata, Column('parents_id', Integer, ForeignKey('parents.id')), Column('children_id', Integer, ForeignKey('children.id')) ) class Parent(Base): __tablename__ = 'parents' id = Column(Integer, primary_key=True) parent_name = Column(String(45)) child_rel = relationship("Child", secondary=Parents2children, backref= "parents_backref") def __init__(self, parent_name=""): self.parent_name=parent_name def __repr__(self): return "<parents(id:'%i', parent_name:'%s')>" % (self.id, self.parent_name) class Child(Base): __tablename__ = 'children' id = Column(Integer, primary_key=True) child_name = Column(String(45)) def __init__(self, child_name=""): self.child_name= child_name def __repr__(self): return "<experiments(id:'%i', child_name:'%s')>" % (self.id, self.child_name) ########################################### def setUp(): global Session engine=create_engine('mysql://root:root@localhost/db_name?charset=utf8', pool_recycle=3600,echo=False) Session=sessionmaker(bind=engine) def add_data(): session=Session() name_father1=Parent(parent_name="Richard") name_mother1=Parent(parent_name="Kate") name_daughter1=Child(chil

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