[gpkg] Ensure that tables present in gpkgext_relations can be read

Tables which are listed in gpkgext_relations but NOT gpkg_contents
should be readable as valid layers. The GeoPackage related tables
extension explicitly states that tables in gpkgext_relations are
not required to be listed in gpkg_contents.

autotest/ogr/ogr_gpkg.py

@@ -6951,6 +6951,13 @@ def test_ogr_gpkg_relations(tmp_vsimem, tmp_path):
     assert rel.GetRightMappingTableFields() == ["related_id"]
     assert rel.GetRelatedTableType() == "attributes"
 
+    # ensure that the mapping table, which is present in gpkgext_relations but
+    # NOT gpkg_contents can be opened as a layer
+    lyr = ds.GetLayer("my_mapping_table")
+    assert lyr is not None
+    assert lyr.GetLayerDefn().GetFieldDefn(0).GetName() == "base_id"
+    assert lyr.GetLayerDefn().GetFieldDefn(1).GetName() == "related_id"
+
     lyr = ds.GetLayer("a")
     lyr.Rename("a_renamed")
     lyr.AlterFieldDefn(
@@ -7212,6 +7219,21 @@ def get_query_row_count(query):
         )
         == 1
     )
+    # force delete from gpkg_contents, and then ensure that we CAN successfully
+    # load layers which are present ONLY in gpkgext_relations but NOT
+    # gpkg_contents (i.e. datasources which follow the Related Tables specification
+    # exactly)
+    ds.ExecuteSQL(
+        "DELETE FROM gpkg_contents WHERE table_name='origin_table_dest_table'"
+    )
+
+    ds = gdal.OpenEx(filename, gdal.OF_VECTOR | gdal.OF_UPDATE)
+    # ensure that the mapping table, which is present in gpkgext_relations but
+    # NOT gpkg_contents can be opened as a layer
+    lyr = ds.GetLayer("origin_table_dest_table")
+    assert lyr is not None
+    assert lyr.GetLayerDefn().GetFieldDefn(0).GetName() == "base_id"
+    assert lyr.GetLayerDefn().GetFieldDefn(1).GetName() == "related_id"
 
     lyr = ds.CreateLayer("origin_table2", geom_type=ogr.wkbNone)
     fld_defn = ogr.FieldDefn("o_pkey", ogr.OFTInteger)

ogr/ogrsf_frmts/gpkg/ogrgeopackagedatasource.cpp

@@ -1598,6 +1598,7 @@ int GDALGeoPackageDataset::Open(GDALOpenInfo *poOpenInfo,
     CheckUnknownExtensions();
 
     int bRet = FALSE;
+    bool bHasGPKGExtRelations = false;
     if (poOpenInfo->nOpenFlags & GDAL_OF_VECTOR)
     {
         m_bHasGPKGGeometryColumns =
@@ -1606,6 +1607,7 @@ int GDALGeoPackageDataset::Open(GDALOpenInfo *poOpenInfo,
                           "name = 'gpkg_geometry_columns' AND "
                           "type IN ('table', 'view')",
                           nullptr) == 1;
+        bHasGPKGExtRelations = HasGpkgextRelationsTable();
     }
     if (m_bHasGPKGGeometryColumns)
     {
@@ -1645,6 +1647,18 @@ int GDALGeoPackageDataset::Open(GDALOpenInfo *poOpenInfo,
             bHasASpatialOrAttributes =
                 (oResultTable && oResultTable->RowCount() == 1);
         }
+        if (bHasGPKGExtRelations)
+        {
+            osSQL += "UNION ALL "
+                     "SELECT mapping_table_name, mapping_table_name, 0 as "
+                     "is_spatial, NULL, NULL, 0, 0, 0 AS "
+                     "xmin, 0 AS ymin, 0 AS xmax, 0 AS ymax, 0 AS "
+                     "is_in_gpkg_contents, 'table' AS object_type "
+                     "FROM gpkgext_relations WHERE "
+                     "lower(mapping_table_name) NOT IN (SELECT "
+                     "lower(table_name) FROM "
+                     "gpkg_contents)";
+        }
         if (EQUAL(pszListAllTables, "YES") ||
             (!bHasASpatialOrAttributes && EQUAL(pszListAllTables, "AUTO")))
         {
@@ -1663,6 +1677,12 @@ int GDALGeoPackageDataset::Open(GDALOpenInfo *poOpenInfo,
                 "'st_geometry_columns', 'geometry_columns') "
                 "AND lower(name) NOT IN (SELECT lower(table_name) FROM "
                 "gpkg_contents)";
+            if (bHasGPKGExtRelations)
+            {
+                osSQL += " AND lower(name) NOT IN (SELECT "
+                         "lower(mapping_table_name) FROM "
+                         "gpkgext_relations)";
+            }
         }
         const int nTableLimit = GetOGRTableLimit();
         if (nTableLimit > 0)