Fix relations in offline editing

src/core/qgsofflineediting.cpp

@@ -254,6 +254,7 @@ void QgsOfflineEditing::synchronize()
 
       // copy style
       copySymbology( offlineLayer, remoteLayer );
+      updateRelations( offlineLayer, remoteLayer );
 
       // apply layer edit log
       QString qgisLayerId = layer->id();
@@ -611,6 +612,7 @@ QgsVectorLayer* QgsOfflineEditing::copyVectorLayer( QgsVectorLayer* layer, sqlit
         }
       }
 
+      updateRelations( layer, newLayer );
       // copy features
       newLayer->startEditing();
       QgsFeature f;
@@ -898,6 +900,29 @@ void QgsOfflineEditing::copySymbology( QgsVectorLayer* sourceLayer, QgsVectorLay
   }
 }
 
+void QgsOfflineEditing::updateRelations( QgsVectorLayer* sourceLayer, QgsVectorLayer* targetLayer )
+{
+  QgsRelationManager* relationManager = QgsProject::instance()->relationManager();
+  QList<QgsRelation> relations;
+  relations = relationManager->referencedRelations( sourceLayer );
+
+  Q_FOREACH ( QgsRelation relation, relations )
+  {
+    relationManager->removeRelation( relation );
+    relation.setReferencedLayer( targetLayer->id() );
+    relationManager->addRelation( relation );
+  }
+
+  relations = relationManager->referencingRelations( sourceLayer );
+
+  Q_FOREACH ( QgsRelation relation, relations )
+  {
+    relationManager->removeRelation( relation );
+    relation.setReferencingLayer( targetLayer->id() );
+    relationManager->addRelation( relation );
+  }
+}
+
 // NOTE: use this to map column indices in case the remote geometry column is not last
 QMap<int, int> QgsOfflineEditing::attributeLookup( QgsVectorLayer* offlineLayer, QgsVectorLayer* remoteLayer )
 {

src/core/qgsofflineediting.h

@@ -108,6 +108,11 @@ class CORE_EXPORT QgsOfflineEditing : public QObject
     void applyGeometryChanges( QgsVectorLayer* remoteLayer, sqlite3* db, int layerId, int commitNo );
     void updateFidLookup( QgsVectorLayer* remoteLayer, sqlite3* db, int layerId );
     void copySymbology( QgsVectorLayer* sourceLayer, QgsVectorLayer* targetLayer );
+
+    /**
+     * Updates all relations that reference or are referenced by the source layer to the targetLayer.
+     */
+    void updateRelations( QgsVectorLayer* sourceLayer, QgsVectorLayer* targetLayer );
     QMap<int, int> attributeLookup( QgsVectorLayer* offlineLayer, QgsVectorLayer* remoteLayer );
 
     void showWarning( const QString& message );