SQLAlchemy - Mapping self-referential relationship as one to many (declarative form)

I want to map a Tag entity using declarative method with SQLAlchemy. A tag can have a parent (another Tag).

I have:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, primary_key=True)
    label = Column(String)

    def __init__(self, label, parentTag=None):
        self.label = label

How can I add the "parent" relationship?

You add a ForeignKey referencing the parent, and then create a relationship that specifies the direction via remote_side. This is documented under adjacency list relationships. For declarative you'd do something like this:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, primary_key=True)
    label = Column(String)
    parent_id = Column(Integer, ForeignKey('tag.id'))

    parent = relationship('Tag', remote_side=[id])

If you want the reverse relation also, add backref='children' to the relationship definition.

If you need children, you need to use uselist:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, primary_key=True)
    label = Column(String)
    child_id = Column(Integer, ForeignKey('tag.id'))

    children = relation('Tag', remote_side=[id], uselist=True)

SQLAlchemy - Mapping self-referential relationship as one to many (declarative form)

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