Add items for project map layers and relations to expression builder

Allows easy insertion of map layer IDs and relation IDs into
expressions. Numerous expression functions now utilise these,
so it makes sense to allow them to be easily inserted.

Fix #11680, #16879

Sponsored by Andreas Neumann

resources/function_help/json/Map Layers

@@ -0,0 +1,5 @@
+{
+  "name": "Map Layers",
+  "type": "group",
+  "description": "Contains a list of map layers available in the current project."
+}

resources/function_help/json/Relations

@@ -0,0 +1,5 @@
+{
+  "name": "Relations",
+  "type": "group",
+  "description": "Contains a list of relations available in the current project."
+}

src/gui/qgsexpressionbuilderwidget.cpp

@@ -25,6 +25,9 @@
 #include "qgsfeatureiterator.h"
 #include "qgsvectorlayer.h"
 #include "qgssettings.h"
+#include "qgsproject.h"
+#include "qgsrelationmanager.h"
+#include "qgsrelation.h"
 
 #include <QMenu>
 #include <QFile>
@@ -440,6 +443,26 @@ void QgsExpressionBuilderWidget::loadRecent( const QString &collection )
   }
 }
 
+void QgsExpressionBuilderWidget::loadLayers()
+{
+  QMap<QString, QgsMapLayer *> layers = QgsProject::instance()->mapLayers();
+  QMap<QString, QgsMapLayer *>::const_iterator layerIt = layers.constBegin();
+  for ( ; layerIt != layers.constEnd(); ++layerIt )
+  {
+    registerItemForAllGroups( QStringList() << tr( "Map Layers" ), layerIt.value()->name(), QStringLiteral( "'%1'" ).arg( layerIt.key() ), formatLayerHelp( layerIt.value() ) );
+  }
+}
+
+void QgsExpressionBuilderWidget::loadRelations()
+{
+  QMap<QString, QgsRelation> relations = QgsProject::instance()->relationManager()->relations();
+  QMap<QString, QgsRelation>::const_iterator relIt = relations.constBegin();
+  for ( ; relIt != relations.constEnd(); ++relIt )
+  {
+    registerItemForAllGroups( QStringList() << tr( "Relations" ), relIt->name(), QStringLiteral( "'%1'" ).arg( relIt->id() ), formatRelationHelp( relIt.value() ) );
+  }
+}
+
 void QgsExpressionBuilderWidget::updateFunctionTree()
 {
   mModel->clear();
@@ -495,6 +518,12 @@ void QgsExpressionBuilderWidget::updateFunctionTree()
     registerItemForAllGroups( func->groups(), func->name(), ' ' + name + ' ', func->helpText() );
   }
 
+  // load relation names
+  loadRelations();
+
+  // load layer IDs
+  loadLayers();
+
   loadExpressionContext();
 }
 
@@ -614,6 +643,20 @@ void QgsExpressionBuilderWidget::registerItemForAllGroups( const QStringList &gr
   }
 }
 
+QString QgsExpressionBuilderWidget::formatRelationHelp( const QgsRelation &relation ) const
+{
+  QString text = QStringLiteral( "<p>%1</p>" ).arg( tr( "Inserts the relation ID for the relation named '%1'." ).arg( relation.name() ) );
+  text.append( QStringLiteral( "<p>%1</p>" ).arg( tr( "Current value: '%1'" ).arg( relation.id() ) ) );
+  return text;
+}
+
+QString QgsExpressionBuilderWidget::formatLayerHelp( const QgsMapLayer *layer ) const
+{
+  QString text = QStringLiteral( "<p>%1</p>" ).arg( tr( "Inserts the layer ID for the layer named '%1'." ).arg( layer->name() ) );
+  text.append( QStringLiteral( "<p>%1</p>" ).arg( tr( "Current value: '%1'" ).arg( layer->id() ) ) );
+  return text;
+}
+
 void QgsExpressionBuilderWidget::showEvent( QShowEvent *e )
 {
   QWidget::showEvent( e );

src/gui/qgsexpressionbuilderwidget.h

@@ -31,6 +31,7 @@
 
 class QgsFields;
 class QgsExpressionHighlighter;
+class QgsRelation;
 
 /** \ingroup gui
  * An expression item that can be used in the QgsExpressionBuilderWidget tree.
@@ -286,6 +287,12 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
 
     void loadExpressionContext();
 
+    //! Loads current project relations names/id into the expression help tree
+    void loadRelations();
+
+    //! Loads current project layer names/ids into the expression help tree
+    void loadLayers();
+
     /** Registers a node item for the expression builder, adding multiple items when the function exists in multiple groups
       * \param groups The groups the item will be show in the tree view.  If a group doesn't exist it will be created.
       * \param label The label that is show to the user for the item in the tree.
@@ -300,6 +307,16 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
                                    QgsExpressionItem::ItemType type = QgsExpressionItem::ExpressionNode,
                                    bool highlightedItem = false, int sortOrder = 1 );
 
+    /**
+     * Returns a HTML formatted string for use as a \a relation item help.
+     */
+    QString formatRelationHelp( const QgsRelation &relation ) const;
+
+    /**
+     * Returns a HTML formatted string for use as a \a layer item help.
+     */
+    QString formatLayerHelp( const QgsMapLayer *layer ) const;
+
     bool mAutoSave;
     QString mFunctionsPath;
     QgsVectorLayer *mLayer = nullptr;