In SQLAlchemy, how do I create a unique pair?

Question

class PostsSubscribe(Base):
    __tablename__ = 'posts_subscribe'
    id = Column(Integer, primary_key = True)
    post_id = Column(Integer, ForeignKey('posts_posts.id'), nullable=False)
    persona_id = Column(Integer, ForeignKey('personas_personas.id'), nullable=False)

    UniqueConstraint('post_id', 'persona_id') #this doesn't work.
Base.metadata.create_all(engine)

This is my table so far. As you can see, I'm using the "Declorative" way of defining tables. I want to create a unique key , but my line doesn't work.

How do I create a unique pair?

Answer 1

UniqueConstraint should be not for a model class, but for its table. You can you __table_args__ to do that:

class PostsSubscribe(Base):
    __tablename__ = 'posts_subscribe'
    id = Column(Integer, primary_key = True)
    post_id = Column(Integer, ForeignKey('posts_posts.id'), nullable=False)
    persona_id = Column(Integer, ForeignKey('personas_personas.id'), nullable=False)
    __table_args__ = (UniqueConstraint('post_id', 'persona_id', name='_person_post_uc'),
                     )