examples.adjacency_list.adjacency_list — SQLAlchemy 2.0 Documentation

Release: 2.0.48 | Release Date: March 2, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.adjacency_list.adjacency_list

    from __future__ import annotations

    from typing import Dict
    from typing import Optional

    from sqlalchemy import create_engine
    from sqlalchemy import ForeignKey
    from sqlalchemy import select
    from sqlalchemy.orm import DeclarativeBase
    from sqlalchemy.orm import Mapped
    from sqlalchemy.orm import mapped_column
    from sqlalchemy.orm import MappedAsDataclass
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import selectinload
    from sqlalchemy.orm import Session
    from sqlalchemy.orm.collections import attribute_keyed_dict

    class Base(DeclarativeBase):
        pass

    class TreeNode(MappedAsDataclass, Base):
        __tablename__ = "tree"

        id: Mapped[int] = mapped_column(primary_key=True, init=False)
        parent_id: Mapped[Optional[int]] = mapped_column(
            ForeignKey("tree.id"), init=False
        )
        name: Mapped[str]

        children: Mapped[Dict[str, TreeNode]] = relationship(
            cascade="all, delete-orphan",
            back_populates="parent",
            collection_class=attribute_keyed_dict("name"),
            init=False,
            repr=False,
        )

        parent: Mapped[Optional[TreeNode]] = relationship(
            back_populates="children", remote_side=id, default=None
        )

        def dump(self, _indent: int = 0) -> str:
            return (
                "   " * _indent
                + repr(self)
                + "\n"
                + "".join([c.dump(_indent + 1) for c in self.children.values()])
            )

    if __name__ == "__main__":
        engine = create_engine("sqlite://", echo=True)

        print("Creating Tree Table:")

        Base.metadata.create_all(engine)

        with Session(engine) as session:
            node = TreeNode("rootnode")
            TreeNode("node1", parent=node)
            TreeNode("node3", parent=node)

            node2 = TreeNode("node2")
            TreeNode("subnode1", parent=node2)
            node.children["node2"] = node2
            TreeNode("subnode2", parent=node.children["node2"])

            print(f"Created new tree structure:\n{node.dump()}")

            print("flush + commit:")

            session.add(node)
            session.commit()

            print(f"Tree after save:\n{node.dump()}")

            session.add_all(
                [
                    TreeNode("node4", parent=node),
                    TreeNode("subnode3", parent=node.children["node4"]),
                    TreeNode("subnode4", parent=node.children["node4"]),
                    TreeNode(
                        "subsubnode1",
                        parent=node.children["node4"].children["subnode3"],
                    ),
                ]
            )

            # remove node1 from the parent, which will trigger a delete
            # via the delete-orphan cascade.
            del node.children["node1"]

            print("Removed node1.  flush + commit:")
            session.commit()

            print("Tree after save, will unexpire all nodes:\n")
            print(f"{node.dump()}")

        with Session(engine) as session:
            print(
                "Perform a full select of the root node, eagerly loading "
                "up to a depth of four"
            )
            node = session.scalars(
                select(TreeNode)
                .options(selectinload(TreeNode.children, recursion_depth=4))
                .filter(TreeNode.name == "rootnode")
            ).one()

            print(f"Full Tree:\n{node.dump()}")

            print("Marking root node as deleted, flush + commit:")

            session.delete(node)
            session.commit()