Merge pull request #57817 from qgis/backport-57653-to-queued_ltr_back…

…ports

[Backport queued_ltr_backports] Fix compound key discovery

src/providers/postgres/qgspostgresprovider.cpp

@@ -5184,8 +5184,7 @@ QList<QgsRelation> QgsPostgresProvider::discoverRelations( const QgsVectorLayer
     "       a.attname as column_name, "
     "       fk.constraint_schema, "
     "       referenced_table as table_name, "
-    "       af.attname as column_name, "
-    "       fk.confkey as ordinal_position "
+    "       af.attname as column_name "
     "FROM foreign_keys fk "
     "JOIN pg_attribute af ON af.attnum = fk.confkey "
     "AND af.attrelid = fk.confrelid "
@@ -5219,13 +5218,11 @@ QList<QgsRelation> QgsPostgresProvider::discoverRelations( const QgsVectorLayer
       refSchema = refSchema.mid( 1, refSchema.length() - 2 );
     }
     const QString refColumn = sqlResult.PQgetvalue( row, 4 );
-    const QString position = sqlResult.PQgetvalue( row, 5 );
+    // try to find if we have layers for the referenced table
     const QList<QgsVectorLayer *> foundLayers = searchLayers( layers, mUri.connectionInfo( false ), refSchema, refTable );
-    if ( ( position == QLatin1String( "1" ) ) || ( !refTableFound.contains( refTable ) ) )
+    if ( !refTableFound.contains( refTable ) )
     {
-      // first reference field => try to find if we have layers for the referenced table
-      const auto constFoundLayers = foundLayers;
-      for ( const QgsVectorLayer *foundLayer : constFoundLayers )
+      for ( const QgsVectorLayer *foundLayer : foundLayers )
       {
         QgsRelation relation;
         relation.setName( name );

tests/src/python/test_provider_postgres.py

@@ -2511,6 +2511,16 @@ def testValidLayerDiscoverRelations(self):
             vl
         ]
 
+        for lyr in vls:
+            self.assertTrue(lyr.isValid())
+            QgsProject.instance().addMapLayer(lyr)
+        relations = vl.dataProvider().discoverRelations(vl, vls)
+        self.assertEqual(len(relations), 2)
+        for i, r in enumerate(relations):
+            self.assertEqual(r.referencedLayer(), vls[i])
+        self.assertEqual(len(relations[0].fieldPairs()), 1)
+        self.assertEqual(len(relations[1].fieldPairs()), 1)
+
     def testValidLayerDiscoverRelationsComposite(self):
         """
         Test implicit relations that can be discovered between tables, based on declared composite foreign keys.

tests/testdata/provider/testdata_pg.sql

@@ -787,5 +787,7 @@ CREATE TABLE qgis_test.referencing_layer_composite(
   fk_ref_4 integer,
     CONSTRAINT fk_ref_3_4
       FOREIGN KEY (fk_ref_3, fk_ref_4)
-      REFERENCES qgis_test.referenced_layer_composite(pk_ref_3, pk_ref_4)
+      -- NOTE: referenced cols are given in reverse order to guard
+      --       against issue GH-56420
+      REFERENCES qgis_test.referenced_layer_composite(pk_ref_4, pk_ref_3)
 );