"""
Illustrates an explicit way to persist an XML document expressed using
ElementTree.
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 pickle_type.py 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
class.
"""
# PART I - Imports/Configuration
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 String
from sqlalchemy import Table
from sqlalchemy import Unicode
from sqlalchemy.orm import aliased
from sqlalchemy.orm import lazyload
from sqlalchemy.orm import mapper
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
e = create_engine("sqlite://")
mapper_registry = registry()
# PART II - Table Metadata
# stores a top level record of an XML document.
documents = Table(
"documents",
mapper_registry.metadata,
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(
"elements",
mapper_registry.metadata,
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(
"attributes",
mapper_registry.metadata,
Column(
"element_id",
Integer,
ForeignKey("elements.element_id"),
primary_key=True,
),
Column("name", Unicode(100), nullable=False, primary_key=True),
Column("value", Unicode(255)),
)
mapper_registry.metadata.create_all(e)
# PART III - Model
# our document class. contains a string name,
# and the ElementTree root element.
class Document:
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:
pass
# Attribute class. also internal, this will represent the key/value attributes
# stored for a particular Node.
class _Attribute:
def __init__(self, name, value):
self.name = name
self.value = value
# setup mappers. Document will eagerly load a list of _Node objects.
mapper(
Document,
documents,
properties={"_root": relationship(_Node, lazy="joined", cascade="all")},
)
mapper(
_Node,
elements,
properties={
"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:
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)
else:
elem = ElementTree.Element(node.tag)
elem.text = node.text
elem.tail = node.tail
for attr in node.attributes:
elem.attrib[attr.name] = 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)
session.commit()
print("Done.")
print("\nFull text of document 'text.xml':", line)
document = session.query(Document).filter_by(filename="test.xml").first()
ElementTree.dump(document.element)
# PART VI - Searching for Paths
# manually search for a document which contains "/somefile/header/field1:hi"
root = aliased(_Node)
child_node = aliased(_Node)
grandchild_node = aliased(_Node)
d = (
session.query(Document)
.join(Document._root.of_type(root))
.filter(root.tag == "somefile")
.join(root.children.of_type(child_node))
.filter(child_node.tag == "header")
.join(child_node.children.of_type(grandchild_node))
.filter(
and_(grandchild_node.tag == "field1", grandchild_node.text == "hi")
)
.one()
)
ElementTree.dump(d.element)
# generalize the above approach into an extremely impoverished xpath function:
def find_document(path, compareto):
query = session.query(Document)
attribute = Document._root
for i, match in enumerate(
re.finditer(r"/([\w_]+)(?:\[@([\w_]+)(?:=(.*))?\])?", path)
):
(token, attrname, attrvalue) = match.group(1, 2, 3)
target_node = aliased(_Node)
query = query.join(attribute.of_type(target_node)).filter(
target_node.tag == token
)
attribute = target_node.children
if attrname:
attribute_entity = aliased(_Attribute)
if attrvalue:
query = query.join(
target_node.attributes.of_type(attribute_entity)
).filter(
and_(
attribute_entity.name == attrname,
attribute_entity.value == attrvalue,
)
)
else:
query = query.join(
target_node.attributes.of_type(attribute_entity)
).filter(attribute_entity.name == attrname)
return (
query.options(lazyload(Document._root))
.filter(target_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)])