Don't crash when a relation is created where the field names defined …

…in the relation differ in case from the layer's field names

src/core/qgsrelation.cpp

@@ -233,7 +233,7 @@ QgsFeatureRequest QgsRelation::getReferencedFeatureRequest( const QgsAttributes
 
   for ( const FieldPair &pair : qgis::as_const( d->mFieldPairs ) )
   {
-    int referencingIdx = referencingLayer()->fields().indexFromName( pair.referencingField() );
+    int referencingIdx = referencingLayer()->fields().lookupField( pair.referencingField() );
     conditions << QgsExpression::createFieldEqualityExpression( pair.referencedField(), attributes.at( referencingIdx ) );
   }
 

tests/src/python/test_qgsrelation.py

@@ -147,6 +147,21 @@ def test_getReferencedFeature(self):
         assert f.isValid()
         assert f[0] == 'foo'
 
+        # try mixing up the field pair field name cases -- we should be tolerant to this
+        rel2 = QgsRelation()
+        rel2.setId('rel1')
+        rel2.setName('Relation Number One')
+        rel2.setReferencingLayer(self.referencingLayer.id())
+        rel2.setReferencedLayer(self.referencedLayer.id())
+        rel2.addFieldPair('ForeignKey', 'Y')
+
+        feat = next(self.referencingLayer.getFeatures())
+
+        f = rel2.getReferencedFeature(feat)
+
+        assert f.isValid()
+        assert f[0] == 'foo'
+
     def test_fieldPairs(self):
         rel = QgsRelation()