Merged in univerio/sqlalchemy/materialized_paths (pull request #21)

Materialized paths example

doc/build/orm/examples.rst

@@ -52,6 +52,11 @@ Large Collections
 
 .. automodule:: examples.large_collection
 
+Materialized Paths
+------------------
+
+.. automodule:: examples.materialized_paths
+
 Nested Sets
 ------------
 

examples/materialized_paths/__init__.py

@@ -0,0 +1,6 @@
+"""Illustrates the "materialized paths" pattern for hierarchical data using the
+SQLAlchemy ORM.
+
+.. autosource::
+
+"""

examples/materialized_paths/materialized_paths.py

@@ -0,0 +1,125 @@
+"""Illustrates the "materialized paths" pattern.
+
+Materialized paths is a way to represent a tree structure in SQL with fast
+descendant and ancestor queries at the expense of moving nodes (which require
+O(n) UPDATEs in the worst case, where n is the number of nodes in the tree). It
+is a good balance in terms of performance and simplicity between the nested
+sets model and the adjacency list model.
+
+It works by storing all nodes in a table with a path column, containing a
+string of delimited IDs. Think file system paths:
+
+    1
+    1.2
+    1.3
+    1.3.4
+    1.3.5
+    1.3.6
+    1.7
+    1.7.8
+    1.7.9
+    1.7.9.10
+    1.7.11
+
+Descendant queries are simple left-anchored LIKE queries, and ancestors are
+already stored in the path itself. Updates require going through all
+descendants and changing the prefix.
+
+"""
+from sqlalchemy import Column, Integer, String, func, select, create_engine
+from sqlalchemy.orm import remote, foreign, relationship, Session
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.sql.expression import cast
+from sqlalchemy.dialects.postgresql import ARRAY
+
+Base = declarative_base()
+
+
+class Node(Base):
+    __tablename__ = "node"
+
+    id = Column(Integer, primary_key=True, autoincrement=False)
+    path = Column(String(500), nullable=False, index=True)
+
+    # To find the descendants of this node, we look for nodes whose path
+    # starts with this node's path.
+    descendants = relationship(
+        "Node", viewonly=True, order_by=path,
+        primaryjoin=remote(foreign(path)).like(path.concat(".%")))
+
+    # Finding the ancestors is a little bit trickier. We need to create a fake
+    # secondary table since this behaves like a many-to-many join.
+    secondary = select([
+        id.label("id"),
+        func.unnest(cast(func.string_to_array(
+            func.regexp_replace(path, r"\.?\d+$", ""), "."),
+            ARRAY(Integer))).label("ancestor_id")
+    ]).alias()
+    ancestors = relationship("Node", viewonly=True, secondary=secondary,
+                             primaryjoin=id == secondary.c.id,
+                             secondaryjoin=secondary.c.ancestor_id == id,
+                             order_by=path)
+
+    @property
+    def depth(self):
+        return len(self.path.split(".")) - 1
+
+    def __repr__(self):
+        return "Node(id={})".format(self.id)
+
+    def __str__(self):
+        root_depth = self.depth
+        s = [str(self.id)]
+        s.extend(((n.depth - root_depth) * "  " + str(n.id))
+                 for n in self.descendants)
+        return "\n".join(s)
+
+    def move_to(self, new_parent):
+        new_path = new_parent.path + "." + str(self.id)
+        for n in self.descendants:
+            n.path = new_path + n.path[len(self.path):]
+        self.path = new_path
+
+
+if __name__ == "__main__":
+    engine = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
+    Base.metadata.create_all(engine)
+
+    session = Session(engine)
+
+    print("-" * 80)
+    print("create a tree")
+    session.add_all([
+        Node(id=1, path="1"),
+        Node(id=2, path="1.2"),
+        Node(id=3, path="1.3"),
+        Node(id=4, path="1.3.4"),
+        Node(id=5, path="1.3.5"),
+        Node(id=6, path="1.3.6"),
+        Node(id=7, path="1.7"),
+        Node(id=8, path="1.7.8"),
+        Node(id=9, path="1.7.9"),
+        Node(id=10, path="1.7.9.10"),
+        Node(id=11, path="1.7.11"),
+    ])
+    session.flush()
+    print(str(session.query(Node).get(1)))
+
+    print("-" * 80)
+    print("move 7 under 3")
+    session.query(Node).get(7).move_to(session.query(Node).get(3))
+    session.flush()
+    print(str(session.query(Node).get(1)))
+
+    print("-" * 80)
+    print("move 3 under 2")
+    session.query(Node).get(3).move_to(session.query(Node).get(2))
+    session.flush()
+    print(str(session.query(Node).get(1)))
+
+    print("-" * 80)
+    print("find the ancestors of 10")
+    print([n.id for n in session.query(Node).get(10).ancestors])
+
+    session.close()
+    Base.metadata.drop_all(engine)