[feature] Editor widget auto config for foreign keys (#3699)

Automatically pick RelationReference widgets for foreign keys.

Author: pvalsecc

src/gui/editorwidgets/qgsrelationreferencefactory.cpp

@@ -181,3 +181,9 @@ QVariant QgsRelationReferenceFactory::sortValue( QgsVectorLayer* vl, int fieldId
 {
   return representValue( vl, fieldIdx, config, cache, value );
 }
+
+unsigned int QgsRelationReferenceFactory::fieldScore( const QgsVectorLayer* vl, int fieldIdx ) const
+{
+  const QList<QgsRelation> relations = vl->referencingRelations( fieldIdx );
+  return !relations.isEmpty() ? 21 /*A bit stronger than the range widget*/ : 5;
+}

src/gui/editorwidgets/qgsrelationreferencefactory.h

@@ -88,6 +88,8 @@ class GUI_EXPORT QgsRelationReferenceFactory : public QgsEditorWidgetFactory
 
     virtual QHash<const char *, int> supportedWidgetTypes() override;
 
+    virtual unsigned int fieldScore( const QgsVectorLayer* vl, int fieldIdx ) const override;
+
   private:
     QgsAttributeEditorContext mEditorContext;
     QgsMapCanvas* mCanvas;

src/gui/editorwidgets/qgsrelationreferencewidgetwrapper.cpp

@@ -62,7 +62,10 @@ void QgsRelationReferenceWidgetWrapper::initWidget( QWidget* editor )
   }
   mWidget->setAllowAddFeatures( config( QStringLiteral( "AllowAddFeatures" ), false ).toBool() );
 
-  QgsRelation relation = QgsProject::instance()->relationManager()->relation( config( QStringLiteral( "Relation" ) ).toString() );
+  const QVariant relationName = config( QStringLiteral( "Relation" ) );
+  QgsRelation relation = relationName.isValid() ?
+                         QgsProject::instance()->relationManager()->relation( relationName.toString() ) :
+                         layer()->referencingRelations( fieldIdx() )[0];
 
   // If this widget is already embedded by the same relation, reduce functionality
   const QgsAttributeEditorContext* ctx = &context();

tests/src/gui/testqgseditorwidgetregistry.cpp

@@ -16,6 +16,9 @@
 
 #include "qgseditorwidgetregistry.h"
 #include "qgseditorwidgetautoconf.h"
+#include "qgsproject.h"
+#include "qgsrelationmanager.h"
+#include "qgsmaplayerregistry.h"
 
 
 class TestQgsEditorWidgetRegistry: public QObject
@@ -109,6 +112,30 @@ class TestQgsEditorWidgetRegistry: public QObject
       QCOMPARE( setup.config().count(), 0 );
     }
 
+    void referencedLayers()
+    {
+      //build two layers
+      QgsVectorLayer vl1( QStringLiteral( "LineString?crs=epsg:3111&field=pk:int&field=name:string&field=fk:int" ), QStringLiteral( "vl1" ), QStringLiteral( "memory" ) );
+      QgsVectorLayer vl2( QStringLiteral( "LineString?crs=epsg:3111&field=pk:int&field=col1:string" ), QStringLiteral( "vl2" ), QStringLiteral( "memory" ) );
+      QgsMapLayerRegistry::instance()->addMapLayer( &vl1, false, false );
+      QgsMapLayerRegistry::instance()->addMapLayer( &vl2, false, false );
+
+      //create a relation between them
+      QgsRelation relation;
+      relation.setRelationId( QStringLiteral( "vl1->vl2" ) );
+      relation.setRelationName( QStringLiteral( "vl1->vl2" ) );
+      relation.setReferencingLayer( vl1.id() );
+      relation.setReferencedLayer( vl2.id() );
+      relation.addFieldPair( "fk", "pk" );
+      QVERIFY( relation.isValid() );
+      QgsProject::instance()->relationManager()->addRelation( relation );
+
+      //check the guessed editor widget type for vl1.fk is RelationReference
+      const QgsEditorWidgetSetup setup = QgsEditorWidgetRegistry::instance()->findBest( &vl1, QStringLiteral( "fk" ) );
+      QCOMPARE( setup.type(), QString( "RelationReference" ) );
+      QCOMPARE( setup.config(), QgsEditorWidgetConfig() );
+    }
+
     void typeFromPlugin()
     {
       const QgsVectorLayer vl( QStringLiteral( "LineString?crs=epsg:3111&field=pk:int&field=special:string" ), QStringLiteral( "vl" ), QStringLiteral( "memory" ) );