[expressions] Fix fetching joined column refs when expression is

not prepared (fix #14746)

(cherry-picked from febe30d)

src/core/qgsexpression.cpp

@@ -4262,11 +4262,23 @@ QgsExpression::Node*QgsExpression::NodeLiteral::clone() const
 QVariant QgsExpression::NodeColumnRef::eval( QgsExpression *parent, const QgsExpressionContext *context )
 {
   Q_UNUSED( parent );
+  int index = mIndex;
+
+  if ( index < 0 )
+  {
+    // have not yet found field index - first check explicitly set fields collection
+    if ( context && context->hasVariable( QgsExpressionContext::EXPR_FIELDS ) )
+    {
+      QgsFields fields = qvariant_cast<QgsFields>( context->variable( QgsExpressionContext::EXPR_FIELDS ) );
+      index = fields.fieldNameIndex( mName );
+    }
+  }
+
   if ( context && context->hasVariable( QgsExpressionContext::EXPR_FEATURE ) )
   {
     QgsFeature feature = qvariant_cast<QgsFeature>( context->variable( QgsExpressionContext::EXPR_FEATURE ) );
-    if ( mIndex >= 0 )
-      return feature.attribute( mIndex );
+    if ( index >= 0 )
+      return feature.attribute( index );
     else
       return feature.attribute( mName );
   }

tests/src/core/testqgsexpression.cpp

@@ -2036,6 +2036,47 @@ class TestQgsExpression: public QObject
       QgsExpression nodeExpression( "1 IN (1, 2, 3, 4)" );
       QgsExpression nodeExpression2( nodeExpression );
     }
+
+    void test_columnRefUnprepared()
+    {
+      //test retrieving fields from feature when expression is unprepared - explicitly specified fields collection
+      //should take precedence over feature's field collection
+
+      QgsFields fields;
+      fields.append( QgsField( "f1", QVariant::String ) );
+
+      QgsFeature f( 1 );
+      f.setFields( fields );
+
+      //also add a joined field - this will not be available in feature's field collection
+      fields.append( QgsField( "j1", QVariant::String ), QgsFields::OriginJoin, 1 );
+
+      f.setAttributes( QgsAttributes() << QVariant( "f1" ) << QVariant( "j1" ) );
+      f.setValid( true );
+
+      QgsExpression e( "\"f1\"" );
+      QgsExpressionContext context;
+      context.setFeature( f );
+      context.setFields( fields );
+      QVariant result = e.evaluate( &context );
+      QCOMPARE( result.toString(), QString( "f1" ) );
+
+      //test joined field
+      QgsExpression e2( "\"j1\"" );
+      result = e2.evaluate( &context );
+      QCOMPARE( result.toString(), QString( "j1" ) );
+
+      // final test - check that feature's field collection is also used when corresponding field NOT found
+      // in explicitly passed field collection
+      fields.append( QgsField( "f2", QVariant::String ) );
+      f.setFields( fields );
+      f.setAttributes( QgsAttributes() << QVariant( "f1" ) << QVariant( "j1" ) << QVariant( "f2" ) );
+      context.setFeature( f );
+      QgsExpression e3( "\"f2\"" );
+      result = e3.evaluate( &context );
+      QCOMPARE( result.toString(), QString( "f2" ) );
+    }
+
 };
 
 QTEST_MAIN( TestQgsExpression )