on deleting a feature, it deletes the relating children, when "cascad…

…e" parameter is true, the relation strength is a composition.

"cascade" parameter is true on every "normal" delete fired by the user interface. It's not true, on functions like merge feature or offline editing synchronization etc.

python/core/auto_generated/qgsvectorlayer.sip.in

@@ -1259,10 +1259,13 @@ Deletes a vertex from a feature.
 .. versionadded:: 2.14
 %End
 
-    bool deleteSelectedFeatures( int *deletedCount = 0 );
+    bool deleteSelectedFeatures( int *deletedCount = 0, const bool cascade = false );
 %Docstring
 Deletes the selected features
 
+:param deleteCount: The number of successfully deleted features
+:param cascade: If the decendants of the feature should be deleted as well
+
 :return: ``True`` in case of success and ``False`` otherwise
 %End
 
@@ -1941,10 +1944,13 @@ Deletes a list of attribute fields (but does not commit it)
     virtual bool addFeatures( QgsFeatureList &features, QgsFeatureSink::Flags flags = 0 ) ${SIP_FINAL};
 
 
-    bool deleteFeature( QgsFeatureId fid );
+    bool deleteFeature( QgsFeatureId fid, const bool cascade = false );
 %Docstring
 Deletes a feature from the layer (but does not commit it).
 
+:param fid: The feature id to delete
+:param cascade: If the decendants of the feature should be deleted as well
+
 .. note::
 
    Calls to deleteFeature() are only valid for layers in which edits have been enabled
@@ -1953,11 +1959,12 @@ Deletes a feature from the layer (but does not commit it).
    changes can be discarded by calling rollBack().
 %End
 
-    bool deleteFeatures( const QgsFeatureIds &fids );
+    bool deleteFeatures( const QgsFeatureIds &fids, const bool cascade = false );
 %Docstring
 Deletes a set of features from the layer (but does not commit it)
 
 :param fids: The feature ids to delete
+:param cascade: If the decendants of the feature should be deleted as well
 
 :return: ``False`` if the layer is not in edit mode or does not support deleting
          in case of an active transaction depends on the provider implementation

python/core/auto_generated/qgsvectorlayerjoinbuffer.sip.in

@@ -207,7 +207,7 @@ created if its fields are not empty.
 .. versionadded:: 3.0
 %End
 
-    bool deleteFeature( QgsFeatureId fid ) const;
+    bool deleteFeature( QgsFeatureId fid, const bool cascade = false ) const;
 %Docstring
 Deletes a feature from joined layers. The feature id given in
 parameter is the one coming from the target layer.
@@ -220,7 +220,7 @@ parameter is the one coming from the target layer.
 .. versionadded:: 3.0
 %End
 
-    bool deleteFeatures( const QgsFeatureIds &fids ) const;
+    bool deleteFeatures( const QgsFeatureIds &fids, const bool cascade = false ) const;
 %Docstring
 Deletes a list of features from joined layers. Feature ids given
 in a parameter are those coming from the target layer.

src/app/qgisapp.cpp

@@ -9003,7 +9003,7 @@ void QgisApp::deleteSelected( QgsMapLayer *layer, QWidget *parent, bool checkFea
 
   vlayer->beginEditCommand( tr( "Features deleted" ) );
   int deletedCount = 0;
-  if ( !vlayer->deleteSelectedFeatures( &deletedCount ) )
+  if ( !vlayer->deleteSelectedFeatures( &deletedCount, true ) )
   {
     visibleMessageBar()->pushMessage( tr( "Problem deleting features" ),
                                       tr( "A problem occurred during deletion from layer \"%1\". %n feature(s) not deleted.", nullptr, numberOfSelectedFeatures - deletedCount ).arg( vlayer->name() ),

src/app/qgsattributetabledialog.cpp

@@ -864,7 +864,7 @@ void QgsAttributeTableDialog::setFilterExpression( const QString &filterString,
 void QgsAttributeTableDialog::deleteFeature( const QgsFeatureId fid )
 {
   QgsDebugMsg( QStringLiteral( "Delete %1" ).arg( fid ) );
-  mLayer->deleteFeature( fid );
+  mLayer->deleteFeature( fid, true );
 }
 
 void QgsAttributeTableDialog::showContextMenu( QgsActionMenu *menu, const QgsFeatureId fid )

src/core/qgsvectorlayer.cpp

@@ -1140,7 +1140,7 @@ QgsVectorLayer::EditResult QgsVectorLayer::deleteVertex( QgsFeatureId featureId,
 }
 
 
-bool QgsVectorLayer::deleteSelectedFeatures( int *deletedCount )
+bool QgsVectorLayer::deleteSelectedFeatures( int *deletedCount, const bool cascade )
 {
   if ( !mValid || !mDataProvider || !( mDataProvider->capabilities() & QgsVectorDataProvider::DeleteFeatures ) )
   {
@@ -1159,7 +1159,7 @@ bool QgsVectorLayer::deleteSelectedFeatures( int *deletedCount )
   const auto constSelectedFeatures = selectedFeatures;
   for ( QgsFeatureId fid : constSelectedFeatures )
   {
-    deleted += deleteFeature( fid );  // removes from selection
+    deleted += deleteFeature( fid, cascade );  // removes from selection
   }
 
   triggerRepaint();
@@ -3164,25 +3164,51 @@ bool QgsVectorLayer::deleteAttributes( const QList<int> &attrs )
   return deleted;
 }
 
-bool QgsVectorLayer::deleteFeatureWithDependencies( QgsFeatureId fid )
+bool QgsVectorLayer::deleteFeatureWithDependencies( QgsFeatureId fid, const bool cascade )
 {
   if ( !mEditBuffer )
     return false;
 
+  if ( cascade )
+  {
+
+    const QList<QgsRelation> relations = QgsProject::instance()->relationManager()->referencedRelations( this );
+
+    for ( const QgsRelation &relation : relations )
+    {
+      //check if composition (and not association)
+      if ( relation.strength() == QgsRelation::Composition )
+      {
+        //get features connected over this relation
+        QgsFeatureIterator relatedFeaturesIt = relation.getRelatedFeatures( getFeature( fid ) );
+        QgsFeatureIds childFeatureIds;
+        QgsFeature childFeature;
+        while ( relatedFeaturesIt.nextFeature( childFeature ) )
+        {
+          childFeatureIds.insert( childFeature.id() );
+        }
+
+        //set childlayer editable
+        relation.referencingLayer()->startEditing();
+        relation.referencingLayer()->deleteFeatures( childFeatureIds );
+      }
+    }
+  }
+
   if ( mJoinBuffer->containsJoins() )
-    mJoinBuffer->deleteFeature( fid );
+    mJoinBuffer->deleteFeature( fid, cascade );
 
   bool res = mEditBuffer->deleteFeature( fid );
 
   return res;
 }
 
-bool QgsVectorLayer::deleteFeature( QgsFeatureId fid )
+bool QgsVectorLayer::deleteFeature( QgsFeatureId fid, const bool cascade )
 {
   if ( !mEditBuffer )
     return false;
 
-  bool res = deleteFeatureWithDependencies( fid );
+  bool res = deleteFeatureWithDependencies( fid, cascade );
 
   if ( res )
   {
@@ -3193,12 +3219,12 @@ bool QgsVectorLayer::deleteFeature( QgsFeatureId fid )
   return res;
 }
 
-bool QgsVectorLayer::deleteFeatures( const QgsFeatureIds &fids )
+bool QgsVectorLayer::deleteFeatures( const QgsFeatureIds &fids, const bool cascade )
 {
   bool res = true;
   const auto constFids = fids;
   for ( QgsFeatureId fid : constFids )
-    res = deleteFeatureWithDependencies( fid ) && res;
+    res = deleteFeatureWithDependencies( fid, cascade ) && res;
 
   if ( res )
   {

src/core/qgsvectorlayer.h

@@ -1278,9 +1278,12 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
 
     /**
      * Deletes the selected features
-     *  \returns TRUE in case of success and FALSE otherwise
+     * \param deleteCount The number of successfully deleted features
+     * \param cascade If the decendants of the feature should be deleted as well
+     *
+     * \returns TRUE in case of success and FALSE otherwise
      */
-    Q_INVOKABLE bool deleteSelectedFeatures( int *deletedCount = nullptr );
+    Q_INVOKABLE bool deleteSelectedFeatures( int *deletedCount = nullptr, const bool cascade = false );
 
     /**
      * Adds a ring to polygon/multipolygon features
@@ -1812,17 +1815,20 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
 
     /**
      * Deletes a feature from the layer (but does not commit it).
+     * \param fid The feature id to delete
+     * \param cascade If the decendants of the feature should be deleted as well
      *
      * \note Calls to deleteFeature() are only valid for layers in which edits have been enabled
      * by a call to startEditing(). Changes made to features using this method are not committed
      * to the underlying data provider until a commitChanges() call is made. Any uncommitted
      * changes can be discarded by calling rollBack().
      */
-    bool deleteFeature( QgsFeatureId fid );
+    bool deleteFeature( QgsFeatureId fid, const bool cascade = false );
 
     /**
      * Deletes a set of features from the layer (but does not commit it)
      * \param fids The feature ids to delete
+     * \param cascade If the decendants of the feature should be deleted as well
      *
      * \returns FALSE if the layer is not in edit mode or does not support deleting
      *         in case of an active transaction depends on the provider implementation
@@ -1832,7 +1838,7 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
      * to the underlying data provider until a commitChanges() call is made. Any uncommitted
      * changes can be discarded by calling rollBack().
      */
-    bool deleteFeatures( const QgsFeatureIds &fids );
+    bool deleteFeatures( const QgsFeatureIds &fids, const bool cascade = false );
 
     /**
      * Attempts to commit to the underlying data provider any buffered changes made since the
@@ -2689,7 +2695,7 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
     //! Read simple labeling from layer's custom properties (QGIS 2.x projects)
     QgsAbstractVectorLayerLabeling *readLabelingFromCustomProperties();
 
-    bool deleteFeatureWithDependencies( QgsFeatureId fid );
+    bool deleteFeatureWithDependencies( QgsFeatureId fid, const bool cascade = false );
 
 #ifdef SIP_RUN
     QgsVectorLayer( const QgsVectorLayer &rhs );

src/core/qgsvectorlayerjoinbuffer.cpp

@@ -663,12 +663,12 @@ bool QgsVectorLayerJoinBuffer::changeAttributeValues( QgsFeatureId fid, const Qg
   return success;
 }
 
-bool QgsVectorLayerJoinBuffer::deleteFeature( QgsFeatureId fid ) const
+bool QgsVectorLayerJoinBuffer::deleteFeature( QgsFeatureId fid, const bool cascade ) const
 {
-  return deleteFeatures( QgsFeatureIds() << fid );
+  return deleteFeatures( QgsFeatureIds() << fid, cascade );
 }
 
-bool QgsVectorLayerJoinBuffer::deleteFeatures( const QgsFeatureIds &fids ) const
+bool QgsVectorLayerJoinBuffer::deleteFeatures( const QgsFeatureIds &fids, const bool cascade ) const
 {
   if ( !containsJoins() )
     return false;
@@ -683,7 +683,7 @@ bool QgsVectorLayerJoinBuffer::deleteFeatures( const QgsFeatureIds &fids ) const
       {
         const QgsFeature joinFeature = joinedFeatureOf( &info, mLayer->getFeature( fid ) );
         if ( joinFeature.isValid() )
-          info.joinLayer()->deleteFeature( joinFeature.id() );
+          info.joinLayer()->deleteFeature( joinFeature.id(), cascade );
       }
     }
   }

src/core/qgsvectorlayerjoinbuffer.h

@@ -202,7 +202,7 @@ class CORE_EXPORT QgsVectorLayerJoinBuffer : public QObject, public QgsFeatureSi
      *
      * \since QGIS 3.0
      */
-    bool deleteFeature( QgsFeatureId fid ) const;
+    bool deleteFeature( QgsFeatureId fid, const bool cascade = false ) const;
 
     /**
      * Deletes a list of features from joined layers. Feature ids given
@@ -214,7 +214,7 @@ class CORE_EXPORT QgsVectorLayerJoinBuffer : public QObject, public QgsFeatureSi
      *
      * \since QGIS 3.0
      */
-    bool deleteFeatures( const QgsFeatureIds &fids ) const;
+    bool deleteFeatures( const QgsFeatureIds &fids, const bool cascade = false ) const;
 
   signals:
 

src/gui/qgsrelationeditorwidget.cpp

@@ -667,10 +667,8 @@ void QgsRelationEditorWidget::deleteFeatures( const QgsFeatureIds &featureids )
     // When deleting a linked feature within an N:M relation,
     // check if the feature is linked to more than just one feature.
     // In case it is linked more than just once, ask the user for confirmation
-    // as it is likely he was not aware of the implications and might either
-    // leave the dataset in a corrupted state (referential integrity) or if
-    // the fk constraint is ON CASCADE DELETE, there may be several linking
-    // entries deleted along.
+    // as it is likely he was not aware of the implications and might delete
+    // there may be several linking entries deleted along.
 
     QgsFeatureRequest deletedFeaturesRequest;
     deletedFeaturesRequest.setFilterFids( featureids );
@@ -732,7 +730,7 @@ void QgsRelationEditorWidget::deleteFeatures( const QgsFeatureIds &featureids )
 
   if ( deleteFeatures )
   {
-    layer->deleteFeatures( featureids );
+    layer->deleteFeatures( featureids, true );
     updateUi();
   }
 }
@@ -793,7 +791,7 @@ void QgsRelationEditorWidget::unlinkFeatures( const QgsFeatureIds &featureids )
       QgsDebugMsgLevel( FID_TO_STRING( f.id() ), 4 );
     }
 
-    mRelation.referencingLayer()->deleteFeatures( fids );
+    mRelation.referencingLayer()->deleteFeatures( fids, false );
 
     updateUi();
   }