Release: 1.3.0b3 beta release | Release Date: February 8, 2019

SQLAlchemy 1.3 Documentation

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Source code for examples.elementtree.adjacency_list

Illustrates an explicit way to persist an XML document expressed using

Each DOM node is stored in an individual
table row, with attributes   represented in a separate table.  The
nodes are associated in a hierarchy using an adjacency list
structure.  A query function is introduced which can search for nodes
along any path with a given   structure of attributes, basically a
(very narrow) subset of xpath.

This example explicitly marshals/unmarshals the ElementTree document into
mapped entities which have their own tables.  Compare to which
uses PickleType to accomplish the same task.  Note that the usage of both
styles of persistence are identical, as is the structure of the main Document


# PART I - Imports/Configuration
from __future__ import print_function

import os
import re
from xml.etree import ElementTree

from sqlalchemy import and_
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy import Unicode
from sqlalchemy.orm import lazyload
from sqlalchemy.orm import mapper
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session

e = create_engine("sqlite://")
meta = MetaData()

# PART II - Table Metadata

# stores a top level record of an XML document.
documents = Table(
    Column("document_id", Integer, primary_key=True),
    Column("filename", String(30), unique=True),
    Column("element_id", Integer, ForeignKey("elements.element_id")),

# stores XML nodes in an adjacency list model.  This corresponds to
# Element and SubElement objects.
elements = Table(
    Column("element_id", Integer, primary_key=True),
    Column("parent_id", Integer, ForeignKey("elements.element_id")),
    Column("tag", Unicode(30), nullable=False),
    Column("text", Unicode),
    Column("tail", Unicode),

# stores attributes.  This corresponds to the dictionary of attributes
# stored by an Element or SubElement.
attributes = Table(
    Column("name", Unicode(100), nullable=False, primary_key=True),
    Column("value", Unicode(255)),


# PART III - Model

# our document class.  contains a string name,
# and the ElementTree root element.

class Document(object):
    def __init__(self, name, element):
        self.filename = name
        self.element = element

# PART IV - Persistence Mapping

# Node class.  a non-public class which will represent the DB-persisted
# Element/SubElement object.  We cannot create mappers for ElementTree elements
# directly because they are at the very least not new-style classes, and also
# may be backed by native implementations. so here we construct an adapter.

class _Node(object):

# Attribute class.  also internal, this will represent the key/value attributes
# stored for a particular Node.

class _Attribute(object):
    def __init__(self, name, value): = name
        self.value = value

# setup mappers.  Document will eagerly load a list of _Node objects.
    properties={"_root": relationship(_Node, lazy="joined", cascade="all")},

        "children": relationship(_Node, cascade="all"),
        # eagerly load attributes
        "attributes": relationship(
            _Attribute, lazy="joined", cascade="all, delete-orphan"

mapper(_Attribute, attributes)

# define marshalling functions that convert from _Node/_Attribute to/from
# ElementTree objects. this will set the ElementTree element as
# "document._element", and append the root _Node object to the "_root" mapped
# collection.

class ElementTreeMarshal(object):
    def __get__(self, document, owner):
        if document is None:
            return self

        if hasattr(document, "_element"):
            return document._element

        def traverse(node, parent=None):
            if parent is not None:
                elem = ElementTree.SubElement(parent, node.tag)
                elem = ElementTree.Element(node.tag)
            elem.text = node.text
            elem.tail = node.tail
            for attr in node.attributes:
                elem.attrib[] = attr.value
            for child in node.children:
                traverse(child, parent=elem)
            return elem

        document._element = ElementTree.ElementTree(traverse(document._root))
        return document._element

    def __set__(self, document, element):
        def traverse(node):
            n = _Node()
            n.tag = str(node.tag)
            n.text = str(node.text)
            n.tail = str(node.tail) if node.tail else None
            n.children = [traverse(n2) for n2 in node]
            n.attributes = [
                _Attribute(str(k), str(v)) for k, v in node.attrib.items()
            return n

        document._root = traverse(element.getroot())
        document._element = element

    def __delete__(self, document):
        del document._element
        document._root = []

# override Document's "element" attribute with the marshaller.
Document.element = ElementTreeMarshal()

# PART V - Basic Persistence Example

line = "\n--------------------------------------------------------"

# save to DB
session = Session(e)

# get ElementTree documents
for file in ("test.xml", "test2.xml", "test3.xml"):
    filename = os.path.join(os.path.dirname(__file__), file)
    doc = ElementTree.parse(filename)
    session.add(Document(file, doc))

print("\nSaving three documents...", line)

print("\nFull text of document 'text.xml':", line)
document = session.query(Document).filter_by(filename="test.xml").first()


# PART VI - Searching for Paths

# manually search for a document which contains "/somefile/header/field1:hi"
d = (
    .join("_root", aliased=True)
    .filter(_Node.tag == "somefile")
    .join("children", aliased=True, from_joinpoint=True)
    .filter(_Node.tag == "header")
    .join("children", aliased=True, from_joinpoint=True)
    .filter(and_(_Node.tag == "field1", _Node.text == "hi"))

# generalize the above approach into an extremely impoverished xpath function:

def find_document(path, compareto):
    j = documents
    prev_elements = None
    query = session.query(Document)
    attribute = "_root"
    for i, match in enumerate(
        re.finditer(r"/([\w_]+)(?:\[@([\w_]+)(?:=(.*))?\])?", path)
        (token, attrname, attrvalue) =, 2, 3)
        query = query.join(
            attribute, aliased=True, from_joinpoint=True
        ).filter(_Node.tag == token)
        attribute = "children"
        if attrname:
            if attrvalue:
                query = query.join(
                    "attributes", aliased=True, from_joinpoint=True
               == attrname,
                        _Attribute.value == attrvalue,
                query = query.join(
                    "attributes", aliased=True, from_joinpoint=True
                ).filter( == attrname)
    return (
        query.options(lazyload("_root")).filter(_Node.text == compareto).all()

for path, compareto in (
    ("/somefile/header/field1", "hi"),
    ("/somefile/field1", "hi"),
    ("/somefile/header/field2", "there"),
    ("/somefile/header/field2[@attr=foo]", "there"),
    print("\nDocuments containing '%s=%s':" % (path, compareto), line)
    print([d.filename for d in find_document(path, compareto)])