cleanup and autogen pk docs

doc/storage.rst

@@ -119,3 +119,35 @@ You may use the following classes as prototypes for the own storages:
 .. autoclass:: AbstractFileStorage
    :members:
    :show-inheritance:
+
+Auto-generated primary keys
+===========================
+
+Both RDBMS and file storages support auto-generated primary keys. For RDBMS
+keys are generated by SQL server (key is auto-incremented integer field). For
+file storages, key are generated as UUIDs.
+
+When object with null primary key field is being saved for the first time,
+SmartObject tries to obtain and set primary key field from storage.
+
+To simplify this, you may create :doc:`SmartObject factory <factory>` with
+*autosave=True* argument: all objects, created by this factory, will be
+auto-saved as soon as created.
+
+.. code:: python
+
+   factory = smartobject.SmartObjectFactory(MyObjClass, autosave=True)
+   # the new object is automatically saved and its primary key is set from the
+   # storage
+   obj = factory.create()
+
+Storage cleanup
+===============
+
+Sometimes storage may contain orphaned objects. To clean them, it has
+*cleanup(pks)* method, where *pks* is a list of object primary keys to leave,
+while all other objects will be removed.
+
+if you use :doc:`SmartObject factory <factory>`, you may use its
+*cleanup(storage_id)* method as well. The method removes all objects from the
+specified storage, except the objects in factory.