examples.elementtree.optimized_al — SQLAlchemy 1.2 Documentation

Release: 1.2.19 | Release Date: April 15, 2019

# SQLAlchemy 1.2 Documentation

# Source code for examples.elementtree.optimized_al

    """Uses the same strategy as
      ``adjacency_list.py``, but associates each   DOM row with its owning
      document row, so that a full document of DOM nodes can be loaded
      using O(1) queries - the construction of the "hierarchy" is performed
      after the load in a non-recursive fashion and is more
      efficient.

    """

    # PART I - Imports/Configuration

    import io
    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://", echo=True)
    meta = MetaData()

    # PART II - Table Metadata

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

    # stores XML nodes in an adjacency list model.  This corresponds to
    # Element and SubElement objects.
    elements = Table(
        "elements",
        meta,
        Column("element_id", Integer, primary_key=True),
        Column("parent_id", Integer, ForeignKey("elements.element_id")),
        Column("document_id", Integer, ForeignKey("documents.document_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",
        meta,
        Column(
            "element_id",
            Integer,
            ForeignKey("elements.element_id"),
            primary_key=True,
        ),
        Column("name", Unicode(100), nullable=False, primary_key=True),
        Column("value", Unicode(255)),
    )

    meta.create_all(e)

    # 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

        def __str__(self):
            buf = io.StringIO()
            self.element.write(buf)
            return buf.getvalue()

    # 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):
        pass

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

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

    # setup mappers.  Document will eagerly load a list of _Node objects.
    # they will be ordered in primary key/insert order, so that we can reconstruct
    # an ElementTree structure from the list.
    mapper(
        Document,
        documents,
        properties={
            "_nodes": relationship(
                _Node, lazy="joined", cascade="all, delete-orphan"
            )
        },
    )

    # the _Node objects change the way they load so that a list of _Nodes will
    # organize themselves hierarchically using the ElementTreeMarshal.  this
    # depends on the ordering of nodes being hierarchical as well; relationship()
    # always applies at least ROWID/primary key ordering to rows which will
    # suffice.
    mapper(
        _Node,
        elements,
        properties={
            "children": relationship(
                _Node, lazy=None
            ),  # doesnt load; used only for the save relationship
            "attributes": relationship(
                _Attribute, lazy="joined", cascade="all, delete-orphan"
            ),  # eagerly load attributes
        },
    )

    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 "_nodes" mapped
    # collection.

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

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

            nodes = {}
            root = None
            for node in document._nodes:
                if node.parent_id is not None:
                    parent = nodes[node.parent_id]
                    elem = ElementTree.SubElement(parent, node.tag)
                    nodes[node.element_id] = elem
                else:
                    parent = None
                    elem = root = ElementTree.Element(node.tag)
                    nodes[node.element_id] = root
                for attr in node.attributes:
                    elem.attrib[attr.name] = attr.value
                elem.text = node.text
                elem.tail = node.tail

            document._element = ElementTree.ElementTree(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)
                document._nodes.append(n)
                n.children = [traverse(n2) for n2 in node]
                n.attributes = [
                    _Attribute(str(k), str(v)) for k, v in node.attrib.items()
                ]
                return n

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

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

    # 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()

    print(document)

    # PART VI - Searching for Paths

    # manually search for a document which contains "/somefile/header/field1:hi"
    print("\nManual search for /somefile/header/field1=='hi':", line)
    d = (
        session.query(Document)
        .join("_nodes", aliased=True)
        .filter(and_(_Node.parent_id == None, _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"))
        .one()
    )
    print(d)

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

    def find_document(path, compareto):
        j = documents
        prev_elements = None
        query = session.query(Document)
        first = True
        for i, match in enumerate(
            re.finditer(r"/([\w_]+)(?:\[@([\w_]+)(?:=(.*))?\])?", path)
        ):
            (token, attrname, attrvalue) = match.group(1, 2, 3)
            if first:
                query = query.join("_nodes", aliased=True).filter(
                    _Node.parent_id == None
                )
                first = False
            else:
                query = query.join("children", aliased=True, from_joinpoint=True)
            query = query.filter(_Node.tag == token)
            if attrname:
                query = query.join("attributes", aliased=True, from_joinpoint=True)
                if attrvalue:
                    query = query.filter(
                        and_(
                            _Attribute.name == attrname,
                            _Attribute.value == attrvalue,
                        )
                    )
                else:
                    query = query.filter(_Attribute.name == attrname)
        return (
            query.options(lazyload("_nodes")).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)])