QgsValueRelationSearchW inherits QgsSearchW

and not QgsDefaultSearchW
also handles properly the (no selection) case

src/gui/editorwidgets/qgsdefaultsearchwidgetwrapper.cpp

@@ -68,10 +68,10 @@ void QgsDefaultSearchWidgetWrapper::setExpression( QString exp )
     str = QString( "%1 %2 '%3'" )
           .arg( QgsExpression::quotedColumnRef( fieldName ),
                 numeric ? "=" : mCaseString,
-                numeric
-                ? exp.replace( "'", "''" )
+                numeric ?
+                exp.replace( '\'', "''" )
                 :
-                "%" + exp.replace( "'", "''" ) + "%" ); // escape quotes
+                '%' + exp.replace( '\'', "''" ) + '%' ); // escape quotes
   }
   mExpression = str;
 }

src/gui/editorwidgets/qgsvaluerelationsearchwidgetwrapper.cpp

@@ -21,11 +21,12 @@
 #include "qgsvectorlayer.h"
 #include "qgsfilterlineedit.h"
 
+#include <QSettings>
 #include <QStringListModel>
 #include <QCompleter>
 
 QgsValueRelationSearchWidgetWrapper::QgsValueRelationSearchWidgetWrapper( QgsVectorLayer* vl, int fieldIdx, QWidget* parent )
-    : QgsDefaultSearchWidgetWrapper( vl, fieldIdx, parent )
+    : QgsSearchWidgetWrapper( vl, fieldIdx, parent )
     , mComboBox( 0 )
     , mListWidget( 0 )
     , mLineEdit( 0 )
@@ -42,7 +43,12 @@ bool QgsValueRelationSearchWidgetWrapper::applyDirectly()
   return true;
 }
 
-QVariant QgsValueRelationSearchWidgetWrapper::value()
+QString QgsValueRelationSearchWidgetWrapper::expression()
+{
+  return mExpression;
+}
+
+QVariant QgsValueRelationSearchWidgetWrapper::value() const
 {
   QVariant v;
 
@@ -83,12 +89,40 @@ QVariant QgsValueRelationSearchWidgetWrapper::value()
   return v;
 }
 
+bool QgsValueRelationSearchWidgetWrapper::valid()
+{
+  return true;
+}
+
 void QgsValueRelationSearchWidgetWrapper::valueChanged()
 {
-  setExpression( value().toString() );
+  QVariant vl = value();
+  QSettings settings;
+  setExpression( vl.isNull() ? settings.value( "qgis/nullValue", "NULL" ).toString() : vl.toString() );
   emit expressionChanged( mExpression );
 }
 
+void QgsValueRelationSearchWidgetWrapper::setExpression( QString exp )
+{
+  QSettings settings;
+  QString nullValue = settings.value( "qgis/nullValue", "NULL" ).toString();
+  QString fieldName = layer()->fields().at( mFieldIdx ).name();
+
+  QString str;
+  if ( exp == nullValue )
+  {
+    str = QString( "%1 IS NULL" ).arg( QgsExpression::quotedColumnRef( fieldName ) );
+  }
+  else
+  {
+    str = QString( "%1 = '%3'" )
+          .arg( QgsExpression::quotedColumnRef( fieldName ),
+                exp.replace( '\'', "''" )
+              );
+  }
+  mExpression = str;
+}
+
 QWidget* QgsValueRelationSearchWidgetWrapper::createWidget( QWidget* parent )
 {
   if ( config( "AllowMulti" ).toBool() )
@@ -99,6 +133,7 @@ QWidget* QgsValueRelationSearchWidgetWrapper::createWidget( QWidget* parent )
   {
     return new QgsFilterLineEdit( parent );
   }
+  else
   {
     return new QComboBox( parent );
   }

src/gui/editorwidgets/qgsvaluerelationsearchwidgetwrapper.h

@@ -16,7 +16,7 @@
 #ifndef QGSVALUERELATIONSEARCHWIDGETWRAPPER_H
 #define QGSVALUERELATIONSEARCHWIDGETWRAPPER_H
 
-#include "qgsdefaultsearchwidgetwrapper.h"
+#include "qgssearchwidgetwrapper.h"
 #include "qgsvaluerelationwidgetwrapper.h"
 
 #include <QComboBox>
@@ -26,26 +26,12 @@
 class QgsValueRelationWidgetFactory;
 
 /**
- * Wraps a value relation widget. This widget will offer a combobox with values from another layer
+ * Wraps a value relation search  widget. This widget will offer a combobox with values from another layer
  * referenced by a foreign key (a constraint may be set but is not required on data level).
- * This is useful for having value lists on a separate layer containing codes and their
- * translation to human readable names.
- *
- * Options:
- *
- * <ul>
- * <li><b>Layer</b> <i>The id of the referenced layer.</i></li>
- * <li><b>Key</b> <i>The key field on the referenced layer (code).</i></li>
- * <li><b>Value</b> <i>The value field on the referenced layer (human readable name).</i></li>
- * <li><b>AllowMulti</b> <i>If set to True, will allow multiple selections. This requires the data type to be a string. This does NOT work with normalized database structures.</i></li>
- * <li><b>AllowNull</b> <i>Will offer NULL as a possible value.</i></li>
- * <li><b>FilterExpression</b> <i>If not empty, will be used as expression. Only if this evaluates to True, the value will be shown.</i></li>
- * <li><b>OrderByValue</b> <i>Will order by value instead of key.</i></li>
- * </ul>
- *
+ * It will be used as a search widget and produces expression to look for in the layer.
  */
 
-class GUI_EXPORT QgsValueRelationSearchWidgetWrapper : public QgsDefaultSearchWidgetWrapper
+class GUI_EXPORT QgsValueRelationSearchWidgetWrapper : public QgsSearchWidgetWrapper
 {
     Q_OBJECT
 
@@ -56,8 +42,9 @@ class GUI_EXPORT QgsValueRelationSearchWidgetWrapper : public QgsDefaultSearchWi
   public:
     explicit QgsValueRelationSearchWidgetWrapper( QgsVectorLayer* vl, int fieldIdx, QWidget* parent = 0 );
     bool applyDirectly() override;
-    QVariant value();
-
+    QString expression() override;
+    bool valid() override;
+    QVariant value() const;
 
   protected:
     QWidget* createWidget( QWidget* parent ) override;
@@ -66,6 +53,9 @@ class GUI_EXPORT QgsValueRelationSearchWidgetWrapper : public QgsDefaultSearchWi
   public slots:
     void valueChanged();
 
+  protected slots:
+    void setExpression( QString exp ) override;
+
   private:
     QComboBox* mComboBox;
     QListWidget* mListWidget;