Better handling of invalid relations ...

- do not add them to the manager dialog
- do not add relations to not existent layers (but keep relations to invalid layers)

python/core/auto_generated/qgsrelation.sip.in

@@ -273,6 +273,7 @@ Returns a list of attributes used to form the referencing fields
     bool isValid() const;
 %Docstring
 Returns the validity of this relation. Don't use the information if it's not valid.
+A relation is considered valid if both referenced and referencig layers are valid.
 
 :return: true if the relation is valid
 %End

python/core/auto_generated/qgsrelationmanager.sip.in

@@ -45,6 +45,8 @@ Gets access to the relations managed by this class.
     void addRelation( const QgsRelation &relation );
 %Docstring
 Add a relation.
+Invalid relations are added only if both referencing layer and referenced
+layer exist.
 
 :param relation: The relation to add.
 %End

src/app/qgsrelationmanagerdialog.cpp

@@ -49,6 +49,9 @@ void QgsRelationManagerDialog::setLayers( const QList< QgsVectorLayer * > &layer
 
 void QgsRelationManagerDialog::addRelation( const QgsRelation &rel )
 {
+  if ( ! rel.isValid() )
+    return;
+
   mRelationsTable->setSortingEnabled( false );
   int row = mRelationsTable->rowCount();
   mRelationsTable->insertRow( row );

src/core/qgsrelation.h

@@ -338,6 +338,7 @@ class CORE_EXPORT QgsRelation
 
     /**
      * Returns the validity of this relation. Don't use the information if it's not valid.
+     * A relation is considered valid if both referenced and referencig layers are valid.
      *
      * \returns true if the relation is valid
      */

src/core/qgsrelationmanager.cpp

@@ -50,6 +50,10 @@ QMap<QString, QgsRelation> QgsRelationManager::relations() const
 
 void QgsRelationManager::addRelation( const QgsRelation &relation )
 {
+  // Do not add relations to layers that do not exist
+  if ( !( relation.referencingLayer() && relation.referencedLayer() ) )
+    return;
+
   mRelations.insert( relation.id(), relation );
 
   if ( mProject )

src/core/qgsrelationmanager.h

@@ -59,6 +59,8 @@ class CORE_EXPORT QgsRelationManager : public QObject
 
     /**
      * Add a relation.
+     * Invalid relations are added only if both referencing layer and referenced
+     * layer exist.
      *
      * \param relation The relation to add.
      */