[OGR] Use OGR_F_SetFieldNull() with GDAL >= 2.2 to avoid GeoJSON fiel…

…ds to be unset (fixes #16812)
qgis · Jul 8, 2017 · fb88071 · fb88071
1 parent f8d50ce
commit fb88071
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diff --git a/src/core/qgsvectorfilewriter.cpp b/src/core/qgsvectorfilewriter.cpp
@@ -1771,7 +1771,19 @@ OGRFeatureH QgsVectorFileWriter::createFeature( QgsFeature& feature )
     int ogrField = mAttrIdxToOgrIdx[ fldIdx ];
 
     if ( !attrValue.isValid() || attrValue.isNull() )
+    {
+// Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
+// whereas previously there was only unset fields. For a GeoJSON output,
+// leaving a field unset will cause it to not appear at all in the output
+// feature.
+// When all features of a layer have a field unset, this would cause the
+// field to not be present at all in the output, and thus on reading to
+// have disappeared. #16812
+#ifdef OGRNullMarker
+      OGR_F_SetFieldNull( poFeature, ogrField );
+#endif
       continue;
+    }
 
     switch ( attrValue.type() )
     {

diff --git a/src/providers/ogr/qgsogrprovider.cpp b/src/providers/ogr/qgsogrprovider.cpp
@@ -1035,7 +1035,18 @@ bool QgsOgrProvider::addFeature( QgsFeature& f )
     QVariant attrVal = attrs.at( targetAttributeId );
     if ( attrVal.isNull() || ( type != OFTString && attrVal.toString().isEmpty() ) )
     {
+// Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
+// whereas previously there was only unset fields. For a GeoJSON output,
+// leaving a field unset will cause it to not appear at all in the output
+// feature.
+// When all features of a layer have a field unset, this would cause the
+// field to not be present at all in the output, and thus on reading to
+// have disappeared. #16812
+#ifdef OGRNullMarker
+      OGR_F_SetFieldNull( feature, targetAttributeId );
+#else
       OGR_F_UnsetField( feature, targetAttributeId );
+#endif
     }
     else
     {
@@ -1323,7 +1334,18 @@ bool QgsOgrProvider::changeAttributeValues( const QgsChangedAttributesMap &attr_
 
       if ( it2->isNull() || ( type != OFTString && it2->toString().isEmpty() ) )
       {
+// Starting with GDAL 2.2, there are 2 concepts: unset fields and null fields
+// whereas previously there was only unset fields. For a GeoJSON output,
+// leaving a field unset will cause it to not appear at all in the output
+// feature.
+// When all features of a layer have a field unset, this would cause the
+// field to not be present at all in the output, and thus on reading to
+// have disappeared. #16812
+#ifdef OGRNullMarker
+        OGR_F_SetFieldNull( of, f );
+#else
         OGR_F_UnsetField( of, f );
+#endif
       }
       else
       {