Hi all,
Below is a write-up of a problem I've encountered using app-schemas when
specifying a mapping for an element whose attributes should be chosen
dynamically. I'm also posting a solution that works for me. I would like to
ask if anyone knows a simpler way to archive the same. Please let me know
if I should include more information.
Kind Regards,
Lazar
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# Geoserver 2.4.4 App-Schema Mapping
## Questions
1. What is the simplest way to generate an element that should sometimes
have an xlink:href attribute and sometimes have an xsi:nil attribute, where
the choice is data-driven?
2. As a last resort, is fiddling with geoserver's private cache of XSD
files an accepted technique for coercing it to produce required output?
## Problem and workaround involving xlink:href and xsi:nil
### Requirement
Generate
<gsmlga:eventProcess xsi:nil="true" nilReason="missing">
or
<gsmlga:eventProcess xlink:href="
http://resource.geoscience.gov.au/classifier/ga/eventprocess/deposition";
xlink:title="deposition">
### Solution
App-schema mapping
<AttributeMapping>
<targetAttribute>gsml:relatedFeature/gsmlga:GeologicHistory/gsml:relatedFeature/gsmlga:GeologicEvent/gsmlga:eventProcess</targetAttribute>
<ClientProperty>
<name>xlink:href</name>
<value>EVENTPROCESS_URI</value>
</ClientProperty>
<ClientProperty>
<name>xlink:title</name>
<value>EVENTPROCESS</value>
</ClientProperty>
</AttributeMapping>
<AttributeMapping>
<targetAttribute>gsml:relatedFeature/gsmlga:GeologicHistory/gsml:relatedFeature/gsmlga:GeologicEvent/gsmlga:eventProcess</targetAttribute>
<encodeIfEmpty>true</encodeIfEmpty>
<ClientProperty>
<name>xsi:nil</name>
<value>if_then_else(isNull(EVENTPROCESS_URI), 'true',
Expression.NIL)</value>
</ClientProperty>
<ClientProperty>
<name>nilReason</name>
<value>if_then_else(isNull(EVENTPROCESS_URI), 'missing',
Expression.NIL)</value>
</ClientProperty>
</AttributeMapping>
Target element is `gsmlgu:GeologicUnit`. The schemas are
xmlns:gml="http://www.opengis.net/gml/3.2";
xmlns:gsml="http://xmlns.geosciml.org/GeoSciML-Core/3.2";
xmlns:gsmlga="http://xmlns.geosciml.org/GeologicAge/3.2";
The above mapping works when `EVENTPROCESS_URI` is not null, but otherwise,
it generates
<gsmlga:eventProcess xlink:href="#null" xsi:nil="true"
nilReason="missing"/>
In fact, even the following mapping
<AttributeMapping>
<targetAttribute>gsml:relatedFeature/gsmlga:GeologicHistory/gsml:relatedFeature/gsmlga:GeologicEvent/gsmlga:eventProcess</targetAttribute>
</AttributeMapping>
generates
<gsmlga:eventProcess xlink:href="#null"/>
Since the above mapping is so minimal, I think that geoserver decides to
insert the xlink:href attribute based on something it sees in the
definition of eventProcess (where else?
it's not the mapping and it's not the data). Here is the definition from
geologicAge.xsd
<element maxOccurs="unbounded" minOccurs="1" name="eventProcess"
nillable="true" type="gml:ReferenceType">
<annotation>
<appinfo>
<gml:targetElement>gsmlga:EventProcessTerm</gml:targetElement>
</appinfo>
<documentation>The eventProcess specifies the process or
processes that occurred during the event. Examples include deposition,
extrusion, intrusion, cooling.</documentation>
</annotation>
</element>
Drilling further into the types, however, shows that xlink:href is not a
required attribute.
When I change this definition, in my app-schema-cache, to
<element maxOccurs="unbounded" minOccurs="1" name="eventProcess"
nillable="true"> <!-- the type is gone -->
<annotation>
<appinfo>
<gml:targetElement>gsmlga:EventProcessTerm</gml:targetElement>
</appinfo>
<documentation>The eventProcess specifies the process or
processes that occurred during the event. Examples include deposition,
extrusion, intrusion, cooling.</documentation>
</annotation>
</element>
the mapping works as required. Now that eventProcess can be of any type,
geoserver cannot infer that it should insert a null xlink:href attribute
into the generated element. Why does it in the first place?
### Note
The problem goes away if the requirements are changed to generate
<gsmlga:eventProcess xlink:href="
http://www.opengis.net/def/nil/OGC/0/missing"; xlink:title="missing"/>
or
<gsmlga:eventProcess xlink:href="
http://resource.geoscience.gov.au/classifier/ga/eventprocess/deposition";
xlink:title="deposition">
and not use xsi:nil attribute at all. The mapping becomes straightforward.
<AttributeMapping>
<targetAttribute>gsml:relatedFeature/gsmlga:GeologicHistory/gsml:relatedFeature/gsmlga:GeologicEvent/gsmlga:eventProcess</targetAttribute>
<ClientProperty>
<name>xlink:href</name>
<value>EVENTPROCESS_URI</value>
</ClientProperty>
<ClientProperty>
<name>xlink:title</name>
<value>EVENTPROCESS</value>
</ClientProperty>
</AttributeMapping>
Kind Regards,
Lazar
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