[layouts] Fix incorrect filename for atlas features when using

@atlas_feature

Fixes #19552

(cherry-picked from 6920102)

Author: nyalldawson

src/core/layout/qgslayoutatlas.cpp

@@ -524,6 +524,12 @@ bool QgsLayoutAtlas::prepareForFeature( const int featureI )
   if ( !mCoverageLayer->getFeatures( QgsFeatureRequest().setFilterFid( mFeatureIds[ featureI ].first ) ).nextFeature( mCurrentFeature ) )
     return false;
 
+  mLayout->reportContext().blockSignals( true ); // setFeature emits changed, we don't want 2 signals
+  mLayout->reportContext().setLayer( mCoverageLayer.get() );
+  mLayout->reportContext().blockSignals( false );
+  mLayout->reportContext().setFeature( mCurrentFeature );
+
+  // must come after we've set the report context feature, or the expression context will have an outdated atlas feature
   QgsExpressionContext expressionContext = createExpressionContext();
 
   // generate filename for current feature
@@ -533,11 +539,6 @@ bool QgsLayoutAtlas::prepareForFeature( const int featureI )
     return false;
   }
 
-  mLayout->reportContext().blockSignals( true ); // setFeature emits changed, we don't want 2 signals
-  mLayout->reportContext().setLayer( mCoverageLayer.get() );
-  mLayout->reportContext().blockSignals( false );
-  mLayout->reportContext().setFeature( mCurrentFeature );
-
   emit featureChanged( mCurrentFeature );
   emit messagePushed( QString( tr( "Atlas feature %1 of %2" ) ).arg( featureI + 1 ).arg( mFeatureIds.size() ) );
 

tests/src/python/test_qgslayoutatlas.py

@@ -368,6 +368,19 @@ def filename_test(self):
             self.assertEqual(self.atlas.currentFilename(), expected)
         self.atlas.endRender()
 
+        # using feature attribute (refs https://issues.qgis.org/issues/19552)
+
+        self.atlas.setFilenameExpression("'output_' || attribute(@atlas_feature,'NAME_1')")
+        expected = ['output_Basse-Normandie',
+                    'output_Bretagne',
+                    'output_Pays de la Loire',
+                    'output_Centre']
+        self.atlas.beginRender()
+        for i in range(0, self.atlas.count()):
+            self.atlas.seekTo(i)
+            self.assertEqual(self.atlas.currentFilename(), expected[i])
+        self.atlas.endRender()
+
     def autoscale_render_test(self):
         self.atlas_map.setExtent(
             QgsRectangle(332719.06221504929, 6765214.5887386119, 560957.85090677091, 6993453.3774303338))