[QGIS-Developer] WFS and NULL values

[Even Rouault]

On mercredi 23 janvier 2019 11:42:19 CET Alessandro Pasotti wrote:
> Thanks for all the comments!
> 
> I understand that it depends on the schema, what confuses me is the
> assertion by Andrea in the (quite old) cited email that  "GML and WFS
> explicitly avoid the use of "xs:nil", I could not find any authoritative
> support for that claim.
> 
> So, if that is not the case, we should probably fix QGIS Server's
> DescribeFeatureType to return something like this (for a nillable element):
> 
> <element type="long" name="id_long" minOccours="0" />
> 
> And for the getFeature response, in case the value is NULL:
> 
> <qgs:id xsi:nil="true" />

If you want to be able to set xsi:nil="true", the XSD element declaration must 
have the nillable="true" attribute.
minOccurs="0" is to allow the element to be completely absent.

> 
> How does it sounds?
> 
> 
> On Wed, Jan 23, 2019 at 11:02 AM Even Rouault <even.rouault at spatialys.com>
> 
> wrote:
> > Recent versions of OGR make a difference between :
> > 
> > - a missing XML element, that will be represented in OGR as a "unset
> > field")
> > (allowed if the schema has minOccurs="0")
> > - and a XML element set to <foo xsi:nil="true"/>, that will be represented
> > in
> > OGR as a NULL field value (allowed if the schema has nillable="true")
> > 
> > ( see https://trac.osgeo.org/gdal/wiki/rfc67_nullfieldvalues )
> > 
> > The same for JSon actually, between a property that doesn't appear at all,
> > or
> > which is set to null.
> > 
> > Example, given foo.json with
> > 
> > { "type": "FeatureCollection",
> > 
> >   "features": [
> >   
> >     { "type": "Feature", "properties": { "a": 1, "null_prop": null },
> > 
> > "geometry": null },
> > 
> >     { "type": "Feature", "properties": { "b": 2 }, "geometry": null }
> >   
> >   ]
> > 
> > }
> > 
> > $ ogr2ogr foo.gml foo.json -f gml
> > 
> > $ cat foo.gml
> > <?xml version="1.0" encoding="utf-8" ?>
> > <ogr:FeatureCollection
> > 
> >      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
> >      xsi:schemaLocation="http://ogr.maptools.org/ foo.xsd"
> >      xmlns:ogr="http://ogr.maptools.org/"
> >      xmlns:gml="http://www.opengis.net/gml">
> >   
> >   <gml:boundedBy><gml:null>missing</gml:null></gml:boundedBy>
> >   
> >   <gml:featureMember>
> >   
> >     <ogr:foo fid="foo.0">
> >     
> >       <ogr:a>1</ogr:a>
> >       <ogr:null_prop xsi:nil="true"/>
> >     
> >     </ogr:foo>
> >   
> >   </gml:featureMember>
> >   <gml:featureMember>
> >   
> >     <ogr:foo fid="foo.1">
> >     
> >       <ogr:b>2</ogr:b>
> >     
> >     </ogr:foo>
> >   
> >   </gml:featureMember>
> > 
> > </ogr:FeatureCollection>
> > 
> > $ ogr2ogr out.json foo.gml
> > 
> > $ ogrinfo foo.gml -al
> > INFO: Open of `foo.gml'
> > 
> >       using driver `GML' successful.
> > 
> > Layer name: foo
> > Geometry: Unknown (any)
> > Feature Count: 2
> > Layer SRS WKT:
> > (unknown)
> > Geometry Column = geometryProperty
> > fid: String (0.0) NOT NULL
> > a: Integer (16.0)
> > null_prop: String (0.0)
> > b: Integer (16.0)
> > OGRFeature(foo):0
> > 
> >   fid (String) = foo.0
> >   a (Integer) = 1
> >   null_prop (String) = (null)
> > 
> > OGRFeature(foo):1
> > 
> >   fid (String) = foo.1
> >   b (Integer) = 2
> > 
> > And yes this is not WFS or GML specific, but XML + XML-Schema generic.
> > 
> > --
> > Spatialys - Geospatial professional services
> > http://www.spatialys.com


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