[Exist-open] XMLDBTransformer

[Wolfgang M. <me...@if...>]

Another new feature which needs some explanation:

to have a simple way to query the database from Cocoon, I wrote an=20
XMLDBTransformer class, which has been inspired by other transformers shi=
pped=20
with Cocoon. The transformer is meant as an easy alternative to XSP pages=
=2E=20
XSP pages have several disadvantages. For example, they need to be compil=
ed,=20
which makes them sensible to changing environments.

The transformer is much simpler: it just understands four tags. Basically=
, the=20
transformer watches out for those known tags, takes some action, and repl=
aces=20
the tags with the results of these actions. Here's a code fragment:

<xmldb:collection xmlns:xmldb=3D"http://exist-db.org/transformer/1.0"
    uri=3D"xmldb:exist:///db">
    <!-- iterate through all rdf:Description elements containing the
         term "computer" -->
    <xmldb:for-each query=3D"//rdf:Description[dc:title &amp;=3D 'compute=
r']"
        from=3D"0" to=3D"9" sort-by=3D"/dc:title">
        <!-- output a book element for each entry -->
        <book>
            <!-- extract the title. There's only one title, so we use
                 select-node -->
            <title><xmldb:select-node query=3D"dc:title/text()"/></title>
            <!-- extract the creators. There's probably more than one,
                 so we use a nested for-each -->
            <xmldb:for-each query=3D"dc:creator/text()">
                <creator><xmldb:current-node/></creator>
            </xmldb:for-each>
        </book>
    </xmldb:for-each>
</xmldb:collection>

Before you can start to query the database, you have to select
a collection with the collection tag. It accepts a standard XMLDB URI in =
its=20
uri attribute. To process a query, you may either use the for-each, or th=
e
select-node tag. The difference should be obvious: for-each iterates over=
 all=20
the results it finds for the given query. The body of the statement will =
be=20
evaluated for each query result. Contrary to that, the select-node tag si=
mply=20
selects the first result it finds. If the result set contains more than o=
ne=20
match, the remaining matches will be ignored.

The current-node tag is used to actually insert the result currently proc=
essed=20
by the for-each tag into the output document. You may restrict the number=
 of=20
iterations for a for-each tag by specifying the from and
to attributes. The sort-by attribute is still experimental: the query res=
ults=20
will be sorted by the specified XPath expression. For each of the results=
,=20
the XPath expression is evaluated and the resulting string value is used =
to=20
sort the query results in ascending order.

As shown above, it is possible to nest multiple for-each or select-node t=
ags.=20
The nested tag will be evaluated relative to the current result node. In =
the=20
example above, the main for-each statement selects all rdf:Description=20
fragments whose title contains the term "computer". During each iteration=
, we=20
further process the current result fragment by using nested for-each and=20
select-node tags to select the title and
creators.

The transformer as well as some examples are in the CVS.

Wolfgang