add method to set extra identifier values to Null and embryon of test

python/core/auto_generated/qgsfeaturefiltermodel.sip.in

@@ -134,6 +134,10 @@ It is normally set to the primary key of the layer.
 The identifier field should be a unique field that can be used to identify individual features.
 It is normally set to the primary key of the layer.
 
+.. note::
+
+   This will also reset identifier fields to NULL
+
 .. versionadded:: 3.10
 %End
 
@@ -166,6 +170,14 @@ still be available in the model.
 Allows specifying one value that does not need to match the filter criteria but will
 still be available in the model.
 
+.. versionadded:: 3.10
+%End
+
+    void setExtraIdentifierValuesToNull();
+%Docstring
+Allows specifying one value that does not need to match the filter criteria but will
+still be available in the model as NULL value(s).
+
 .. versionadded:: 3.10
 %End
 

python/gui/auto_generated/qgsfeaturelistcombobox.sip.in

@@ -112,6 +112,13 @@ identifierField.
 The identifier values of the currently selected feature. A value from the
 identifierFields.
 
+.. versionadded:: 3.10
+%End
+
+    void setIdentifierValuesToNull();
+%Docstring
+Sets the identifier values of the currently selected feature to NULL value(s).
+
 .. versionadded:: 3.10
 %End
 

src/core/qgsfeaturefiltermodel.cpp

@@ -216,7 +216,16 @@ void QgsFeatureFilterModel::updateCompleter()
   QVector<Entry> entries = mGatherer->entries();
 
   if ( mExtraIdentifierValueIndex == -1 )
-    setExtraIdentifierValuesUnguarded( QVariantList() );
+  {
+    QVariantList nullAttributes;
+    for ( const QString &fieldName : mIdentifierFields )
+    {
+      int idx = mSourceLayer->fields().indexOf( fieldName );
+      nullAttributes << QVariant( mSourceLayer->fields().at( idx ).type() );
+    }
+
+    setExtraIdentifierValuesUnguarded( nullAttributes );
+  }
 
   // Only reloading the current entry?
   if ( mGatherer->data().toBool() )
@@ -573,7 +582,7 @@ void QgsFeatureFilterModel::setIdentifierFields( const QStringList &identifierFi
 
   mIdentifierFields = identifierFields;
   emit identifierFieldChanged();
-  setExtraIdentifierValues( QVariantList() );
+  setExtraIdentifierValuesToNull();
 }
 
 void QgsFeatureFilterModel::reload()
@@ -617,3 +626,18 @@ void QgsFeatureFilterModel::setExtraIdentifierValues( const QVariantList &extraI
 
   emit extraIdentifierValueChanged();
 }
+
+void QgsFeatureFilterModel::setExtraIdentifierValuesToNull()
+{
+  QVariantList nullAttributes;
+  if ( mSourceLayer )
+  {
+    for ( const QString &fieldName : mIdentifierFields )
+    {
+      int idx = mSourceLayer->fields().indexOf( fieldName );
+      Q_ASSERT( idx >= 0 );
+      nullAttributes << QVariant( mSourceLayer->fields().at( idx ).type() );
+    }
+  }
+  setExtraIdentifierValues( nullAttributes );
+}

src/core/qgsfeaturefiltermodel.h

@@ -159,6 +159,7 @@ class CORE_EXPORT QgsFeatureFilterModel : public QAbstractItemModel
     /**
      * The identifier field should be a unique field that can be used to identify individual features.
      * It is normally set to the primary key of the layer.
+     * \note This will also reset identifier fields to NULL
      * \since QGIS 3.10
      */
     void setIdentifierFields( const QStringList &identifierFields );
@@ -191,6 +192,13 @@ class CORE_EXPORT QgsFeatureFilterModel : public QAbstractItemModel
      */
     void setExtraIdentifierValues( const QVariantList &extraIdentifierValues );
 
+    /**
+     * Allows specifying one value that does not need to match the filter criteria but will
+     * still be available in the model as NULL value(s).
+     * \since QGIS 3.10
+     */
+    void setExtraIdentifierValuesToNull();
+
     /**
      * The index at which the extra identifier value is available within the model.
      */

src/gui/editorwidgets/qgsrelationreferencewidget.cpp

@@ -206,6 +206,7 @@ void QgsRelationReferenceWidget::setRelation( const QgsRelation &relation, bool
     mReferencedField = relation.fieldPairs().at( 0 ).second;
     if ( mComboBox )
     {
+      mComboBox->setSourceLayer( mReferencedLayer );
       Q_NOWARN_DEPRECATED_PUSH
       mComboBox->setIdentifierField( mReferencedField );
       Q_NOWARN_DEPRECATED_PUSH
@@ -350,9 +351,7 @@ void QgsRelationReferenceWidget::deleteForeignKey()
   }
   else
   {
-    Q_NOWARN_DEPRECATED_PUSH
-    mComboBox->setIdentifierValue( QVariant( QVariant::Int ) );
-    Q_NOWARN_DEPRECATED_POP
+    mComboBox->setIdentifierValuesToNull();
   }
   mRemoveFKButton->setEnabled( false );
   updateAttributeEditorFrame( QgsFeature() );
@@ -386,9 +385,7 @@ void QgsRelationReferenceWidget::showIndeterminateState()
   }
   else
   {
-    Q_NOWARN_DEPRECATED_PUSH
-    whileBlocking( mComboBox )->setIdentifierValue( QVariant() );
-    Q_NOWARN_DEPRECATED_POP
+    whileBlocking( mComboBox )->setIdentifierValuesToNull();
   }
   mRemoveFKButton->setEnabled( false );
   updateAttributeEditorFrame( QgsFeature() );

src/gui/qgsfeaturelistcombobox.cpp

@@ -258,6 +258,11 @@ void QgsFeatureListComboBox::setIdentifierValues( const QVariantList &identifier
   mModel->setExtraIdentifierValues( identifierValues );
 }
 
+void QgsFeatureListComboBox::setIdentifierValuesToNull()
+{
+  mModel->setExtraIdentifierValuesToNull();
+}
+
 QgsFeatureRequest QgsFeatureListComboBox::currentFeatureRequest() const
 {
   if ( mModel->extraIdentifierValues().isEmpty() )

src/gui/qgsfeaturelistcombobox.h

@@ -134,6 +134,12 @@ class GUI_EXPORT QgsFeatureListComboBox : public QComboBox
      */
     void setIdentifierValues( const QVariantList &identifierValues );
 
+    /**
+     * Sets the identifier values of the currently selected feature to NULL value(s).
+     * \since QGIS 3.10
+     */
+    void setIdentifierValuesToNull();
+
     /**
      * Shorthand for getting a feature request to query the currently selected
      * feature.

tests/src/gui/testqgsfeaturelistcombobox.cpp

@@ -43,6 +43,7 @@ class TestQgsFeatureListComboBox : public QObject
 
     void testSetGetLayer();
     void testSetGetForeignKey();
+    void testMultipleForeignKeys();
     void testAllowNull();
     void testValuesAndSelection();
     void nullRepresentation();
@@ -139,6 +140,23 @@ void TestQgsFeatureListComboBox::testSetGetForeignKey()
   Q_NOWARN_DEPRECATED_POP
 }
 
+void TestQgsFeatureListComboBox::testMultipleForeignKeys()
+{
+  std::unique_ptr<QgsFeatureListComboBox> cb( new QgsFeatureListComboBox() );
+
+  QgsApplication::setNullRepresentation( QStringLiteral( "nope" ) );
+
+  QVERIFY( cb->identifierValues().isEmpty() );
+
+  cb->setSourceLayer( mLayer.get() );
+  cb->setIdentifierFields( QStringList() << "material" << "diameter" << "raccord" );
+  cb->setDisplayExpression( "\"material\" || ' ' || \"diameter\" || ' ' || \"raccord\"" );
+  cb->setAllowNull( true );
+
+  cb->setIdentifierValues( QVariantList() << "gold" << 777 << "rush" );
+  QCOMPARE( cb->identifierValues(), QVariantList() << "gold" << 777 << "rush" );
+}
+
 void TestQgsFeatureListComboBox::testAllowNull()
 {
   //QVERIFY( false );

tests/src/gui/testqgsrelationreferencewidget.cpp

@@ -403,9 +403,9 @@ void TestQgsRelationReferenceWidget::testAddEntry()
 
       bool startEditing( QgsVectorLayer * ) const override {return true;}
 
-      bool stopEditing( QgsVectorLayer *, bool = true ) const override {return true;};
+      bool stopEditing( QgsVectorLayer *, bool = true ) const override {return true;}
 
-      bool saveEdits( QgsVectorLayer * ) const override {return true;};
+      bool saveEdits( QgsVectorLayer * ) const override {return true;}
   };
 
   QgsAttributeEditorContext context;