[ogr] Pass default value (-1) for fid columns

src/providers/ogr/qgsogrprovider.cpp

@@ -948,6 +948,10 @@ void QgsOgrProvider::loadFields()
     mAttributeFields.append(
       fidField
     );
+    // Set default value for fid, this is needed because
+    // the attribute form will not accept a NULL value, passing
+    // -1 will delegate to the back-end.
+    mDefaultValues.insert( 0, QStringLiteral( "-1" ) );
   }
 
   for ( int i = 0; i < fdef.GetFieldCount(); ++i )
@@ -1201,6 +1205,18 @@ QVariant QgsOgrProvider::defaultValue( int fieldId ) const
   return resultVar;
 }
 
+QString QgsOgrProvider::defaultValueClause( int fieldIndex ) const
+{
+  QString defVal = mDefaultValues.value( fieldIndex, QString() );
+
+  if ( !providerProperty( EvaluateDefaultValues, false ).toBool() && !defVal.isEmpty() )
+  {
+    return defVal;
+  }
+
+  return QString();
+}
+
 void QgsOgrProvider::updateExtents()
 {
   invalidateCachedExtent( true );

src/providers/ogr/qgsogrprovider.h

@@ -109,6 +109,7 @@ class QgsOgrProvider : public QgsVectorDataProvider
     virtual QgsFields fields() const override;
     virtual QgsRectangle extent() const override;
     QVariant defaultValue( int fieldId ) const override;
+    QString defaultValueClause( int fieldIndex ) const override;
     virtual void updateExtents() override;
     virtual bool addFeatures( QgsFeatureList &flist, QgsFeatureSink::Flags flags = 0 ) override;
     virtual bool deleteFeatures( const QgsFeatureIds &id ) override;