pretend that old migrations (< 259) are on branch "1"

db/migrations/1.259_migrations_reloaded.php

@@ -17,6 +17,9 @@ class MigrationsReloaded extends Migration
                 DROP PRIMARY KEY,
                 ADD PRIMARY KEY (domain, branch)";
         $db->exec($sql);
+
+        $sql = "UPDATE schema_version SET branch = '1' WHERE domain = 'studip'";
+        $db->exec($sql);
     }
 
     public function down()

lib/migrations/DBSchemaVersion.php

@@ -157,7 +157,8 @@ class DBSchemaVersion implements SchemaVersion
     }
 
     /**
-     * Validate correct structure of schema_version table.
+     * Validate correct structure of schema_version table. This
+     * will upgrade the schema from 4.4 style to 5.1 if necessary.
      */
     private function validateSchemaVersion()
     {
@@ -170,10 +171,12 @@ class DBSchemaVersion implements SchemaVersion
                 20201002, 20201103, 202011031, 20210317
             ];
 
+            // drop backported migrations
             $query = "DELETE FROM schema_versions
                       WHERE domain = 'studip' AND version in (?)";
             $db->execute($query, [$backported_migrations]);
 
+            // drop migrations with irregular numbers
             $query = "DELETE FROM schema_versions
                       WHERE domain = 'studip' AND LENGTH(version) > 8";
             $db->exec($query);