[Backport to queued_ltr_backports][Postgres] save of cleared attributes

src/providers/postgres/qgspostgresprovider.cpp

@@ -3021,7 +3021,12 @@ bool QgsPostgresProvider::changeAttributeValues( const QgsChangedAttributesMap &
           sql += delim + QStringLiteral( "%1=" ).arg( quotedIdentifier( fld.name() ) );
           delim = ',';
 
-          if ( fld.typeName() == QLatin1String( "geometry" ) )
+          QString defVal = defaultValueClause( siter.key() );
+          if ( qgsVariantEqual( *siter, defVal ) )
+          {
+            sql += defVal.isNull() ? "NULL" : defVal;
+          }
+          else if ( fld.typeName() == QLatin1String( "geometry" ) )
           {
             sql += QStringLiteral( "%1(%2)" )
                    .arg( connectionRO()->majorVersion() < 2 ? "geomfromewkt" : "st_geomfromewkt",

tests/src/python/test_provider_postgres.py

@@ -2586,6 +2586,57 @@ def _test(vl, extent, ids):
 
         _test(vl, QgsRectangle(-181, -90, 181, 90), [1, 2, 3])  # no use of spatial index currently
 
+    def testChangeAttributeWithDefaultValue(self):
+        """Test that we can change an attribute value with its default value"""
+
+        md = QgsProviderRegistry.instance().providerMetadata("postgres")
+        conn = md.createConnection(self.dbconn, {})
+
+        # Cleanup
+        try:
+            conn.dropVectorTable('qgis_test', 'test_change_att_w_default_value')
+        except QgsProviderConnectionException:
+            pass
+
+        conn.executeSql('''
+        CREATE TABLE "qgis_test"."test_change_att_w_default_value" (
+            id serial primary key,
+            thetext1 character varying(8) DEFAULT NULL::character varying,
+            thetext2 character varying(8) DEFAULT NULL,
+            thetext3 character varying(8) DEFAULT 'blabla',
+            thenumber integer DEFAULT 2+2
+        );''')
+
+        conn.executeSql('''
+        INSERT INTO "qgis_test"."test_change_att_w_default_value" (thetext1,thetext2,thetext3,thenumber) VALUES ('test1','test2','test3',6);''')
+
+        layer = QgsVectorLayer(self.dbconn + ' sslmode=disable key=\'id\'table="qgis_test"."test_change_att_w_default_value" sql=', 'test', 'postgres')
+        self.assertTrue(layer.isValid())
+        self.assertEqual(layer.featureCount(), 1)
+        feat = next(layer.getFeatures())
+        self.assertTrue(feat["thetext1"], "test1")
+        self.assertTrue(feat["thetext2"], "test2")
+        self.assertTrue(feat["thetext3"], "test3")
+        self.assertTrue(feat["thenumber"], 6)
+
+        self.assertEqual(layer.dataProvider().defaultValueClause(1), "NULL::character varying")
+        self.assertEqual(layer.dataProvider().defaultValueClause(2), "NULL::character varying")
+        self.assertEqual(layer.dataProvider().defaultValueClause(3), "'blabla'::character varying")
+        self.assertEqual(layer.dataProvider().defaultValueClause(4), "(2 + 2)")
+
+        layer.startEditing()
+        self.assertTrue(layer.changeAttributeValues(1, {1: "NULL::character varying", 2: "NULL::character varying",
+                                                        3: "'blabla'::character varying", 4: "(2 + 2)"}))
+        self.assertTrue(layer.commitChanges())
+
+        feat = next(layer.getFeatures())
+
+        # print( "hein |{}| expected=|{}| bool={}".format( feat["thetext"], expected, (feat["thetext"] expected) ) )
+        self.assertEqual(feat["thetext1"], NULL)
+        self.assertEqual(feat["thetext2"], NULL)
+        self.assertEqual(feat["thetext3"], "blabla")
+        self.assertEqual(feat["thenumber"], 4)
+
 
 class TestPyQgsPostgresProviderCompoundKey(unittest.TestCase, ProviderTestCase):