Joined fields are updated according to the target field name in form

python/core/qgsvectorlayerjoinbuffer.sip

@@ -99,6 +99,24 @@ Quick way to test if there is any join at all
  :rtype: list of int
 %End
 
+    QList<const QgsVectorLayerJoinInfo *> joinsWhereFieldIsId( const QgsField &field ) const;
+%Docstring
+ Returns joins where the field of a target layer is considered as an id.
+ \param field the field of a target layer
+ :return: a list of vector joins
+.. versionadded:: 3.0
+ :rtype: list of const QgsVectorLayerJoinInfo
+%End
+
+    QgsFeature joinedFeatureOf( const QgsVectorLayerJoinInfo &info, const QgsFeature &feature ) const;
+%Docstring
+ Returns the joined feature corresponding to the feature.
+ \param info the vector join information
+ \param feature the feature of the target layer
+.. versionadded:: 3.0
+ :rtype: QgsFeature
+%End
+
     QgsVectorLayerJoinBuffer *clone() const /Factory/;
 %Docstring
 .. versionadded:: 2.6

python/core/qgsvectorlayerjoininfo.sip

@@ -99,6 +99,15 @@ Returns whether values from the joined layer should be cached in memory to speed
 .. versionadded:: 3.0
 %End
 
+    QString prefixedFieldName( const QgsField &field ) const;
+%Docstring
+ Returns the prefixed name of the field.
+ \param field the field
+ :return: the prefixed name of the field
+.. versionadded:: 3.0
+ :rtype: str
+%End
+
     bool operator==( const QgsVectorLayerJoinInfo &other ) const;
 
     void setJoinFieldNamesSubset( QStringList *fieldNamesSubset /Transfer/ );

src/core/CMakeLists.txt

@@ -286,6 +286,7 @@ SET(QGIS_CORE_SRCS
   qgsvectorlayerfeatureiterator.cpp
   qgsvectorlayerexporter.cpp
   qgsvectorlayerjoinbuffer.cpp
+  qgsvectorlayerjoininfo.cpp
   qgsvectorlayerlabeling.cpp
   qgsvectorlayerlabelprovider.cpp
   qgsvectorlayerrenderer.cpp

src/core/qgsvectorlayerjoinbuffer.cpp

@@ -394,6 +394,44 @@ const QgsVectorLayerJoinInfo *QgsVectorLayerJoinBuffer::joinForFieldIndex( int i
   return &( mVectorJoins[sourceJoinIndex] );
 }
 
+QList<const QgsVectorLayerJoinInfo *> QgsVectorLayerJoinBuffer::joinsWhereFieldIsId( const QgsField &field ) const
+{
+  QList<const QgsVectorLayerJoinInfo *> infos;
+
+  for ( int i = 0; i < mVectorJoins.count(); i++ )
+  {
+    const QgsVectorLayerJoinInfo *info = &( mVectorJoins[i] );
+
+    if ( infos.contains( info ) )
+      continue;
+
+    if ( info->targetFieldName() == field.name() )
+      infos.append( info );
+  }
+
+  return infos;
+}
+
+QgsFeature QgsVectorLayerJoinBuffer::joinedFeatureOf( const QgsVectorLayerJoinInfo &info, const QgsFeature &feature ) const
+{
+  QgsFeature joinedFeature;
+
+  if ( info.joinLayer() )
+  {
+    const QVariant targetValue = feature.attribute( info.targetFieldName() );
+    const QString filter = QString( "\"%1\" = %2" ).arg( info.joinFieldName(), targetValue.toString() );
+
+    QgsFeatureRequest request;
+    request.setFilterExpression( filter );
+    request.setLimit( 1 );
+
+    QgsFeatureIterator it = info.joinLayer()->getFeatures( request );
+    it.nextFeature( joinedFeature );
+  }
+
+  return joinedFeature;
+}
+
 QgsVectorLayerJoinBuffer *QgsVectorLayerJoinBuffer::clone() const
 {
   QgsVectorLayerJoinBuffer *cloned = new QgsVectorLayerJoinBuffer( mLayer );

src/core/qgsvectorlayerjoinbuffer.h

@@ -84,6 +84,20 @@ class CORE_EXPORT QgsVectorLayerJoinBuffer : public QObject
     //! \since QGIS 2.6
     static QVector<int> joinSubsetIndices( QgsVectorLayer *joinLayer, const QStringList &joinFieldsSubset );
 
+    /** Returns joins where the field of a target layer is considered as an id.
+     * \param field the field of a target layer
+     * \returns a list of vector joins
+     * \since QGIS3.0
+     */
+    QList<const QgsVectorLayerJoinInfo *> joinsWhereFieldIsId( const QgsField &field ) const;
+
+    /** Returns the joined feature corresponding to the feature.
+     * \param info the vector join information
+     * \param feature the feature of the target layer
+     * \since QGIS 3.0
+     */
+    QgsFeature joinedFeatureOf( const QgsVectorLayerJoinInfo &info, const QgsFeature &feature ) const;
+
     //! Create a copy of the join buffer
     //! \since QGIS 2.6
     QgsVectorLayerJoinBuffer *clone() const SIP_FACTORY;

src/core/qgsvectorlayerjoininfo.h

@@ -66,6 +66,13 @@ class CORE_EXPORT QgsVectorLayerJoinInfo
      */
     void setDynamicFormEnabled( bool enabled ) { mDynamicForm = enabled; }
 
+    /** Returns the prefixed name of the field.
+     * \param field the field
+     * \returns the prefixed name of the field
+     * \since QGIS 3.0
+     */
+    QString prefixedFieldName( const QgsField &field ) const;
+
     bool operator==( const QgsVectorLayerJoinInfo &other ) const
     {
       return mTargetFieldName == other.mTargetFieldName &&

src/gui/qgsattributeform.cpp

@@ -33,6 +33,8 @@
 #include "qgssettings.h"
 #include "qgsscrollarea.h"
 #include "qgsgui.h"
+#include "qgsvectorlayerjoinbuffer.h"
+#include "qgsvectorlayerutils.h"
 
 #include <QDir>
 #include <QTextStream>
@@ -659,6 +661,9 @@ void QgsAttributeForm::onAttributeChanged( const QVariant &value )
       {
         emit attributeChanged( eww->field().name(), value );
       }
+
+      updateJoinedFields( *eww );
+
       break;
     }
     case MultiEditMode:
@@ -1930,3 +1935,50 @@ void QgsAttributeForm::ContainerInformation::apply( QgsExpressionContext *expres
     isVisible = newVisibility;
   }
 }
+
+QgsFeature QgsAttributeForm::joinedFeature( const QgsVectorLayerJoinInfo &info, const QgsFeature &feature ) const
+{
+  QgsFeature joinedFeature = mLayer->joinBuffer()->joinedFeatureOf( info, feature );
+
+  if ( !joinedFeature.isValid() )
+    joinedFeature = QgsVectorLayerUtils::createFeature( info.joinLayer(), QgsGeometry(), QgsAttributeMap() );
+
+  return joinedFeature;
+}
+
+void QgsAttributeForm::updateJoinedFields( const QgsEditorWidgetWrapper &eww )
+{
+  QgsFeature formFeature;
+  QgsField field = eww.layer()->fields().field( eww.fieldIdx() );
+  QList<const QgsVectorLayerJoinInfo *> infos = eww.layer()->joinBuffer()->joinsWhereFieldIsId( field );
+
+  if ( infos.count() == 0 || !currentFormFeature( formFeature ) )
+    return;
+
+  Q_FOREACH ( const QgsVectorLayerJoinInfo *info, infos )
+  {
+    if ( !info->isDynamicFormEnabled() )
+      continue;
+
+    QgsFeature joinFeature = joinedFeature( *info, formFeature );
+
+    QStringList *subsetFields = info->joinFieldNamesSubset();
+    if ( subsetFields )
+    {
+      Q_FOREACH ( const QString &field, *subsetFields )
+      {
+        QString prefixedName = info->prefixedFieldName( field );
+        changeAttribute( prefixedName, joinFeature.attribute( field ) );
+      }
+    }
+    else
+    {
+      for ( int i = 0; i < joinFeature.fields().count(); i++ )
+      {
+        QgsField field = joinFeature.fields().field( i );
+        QString prefixedName = info->prefixedFieldName( field );
+        changeAttribute( prefixedName, joinFeature.attribute( field.name() ) );
+      }
+    }
+  }
+}

src/gui/qgsattributeform.h

@@ -273,6 +273,10 @@ class GUI_EXPORT QgsAttributeForm : public QWidget
 
     void initPython();
 
+    QgsFeature joinedFeature( const QgsVectorLayerJoinInfo &info, const QgsFeature &feature ) const;
+
+    void updateJoinedFields( const QgsEditorWidgetWrapper &eww );
+
     struct WidgetInfo
     {
       WidgetInfo()