Hello, Here follows Common Lisp code, using a datastore library named bknr.datastore (ql:quickload :bknr.datastore) (defclass userdata (bknr.datastore:store-object) ((email :initarg :email :index-type bknr.indices:unique-index :index-values all-users) (name :initarg :name)) (:metaclass bknr.datastore:persistent-class)) (let ((object-subsystem (make-instance 'bknr.datastore:store-object-subsystem))) (make-instance 'bknr.datastore:store :directory *default-pathname-defaults* :subsystems (list object-subsystem))) (make-instance 'userdata :email "m@b.com" :name "Maria") (make-instance 'userdata :email "k@g.com" :name "Katerina") (setf (slot-value (first (all-users)) 'name) "Elena") ; Attempt to modify persistent slot NAME of #<USERDATA ID: 0> outside of a transaction ; [Condition of type BKNR.DATASTORE::PERSISTENT-SLOT-MODIFIED-OUTSIDE-OF-TRANSACTION] (bknr.datastore:with-transaction () (setf (slot-value (first (all-users)) 'name) "Elena")) ; runs successfully, the above wrap in with-transaction () is necessary I use the Slime inspector to view the datastore, so in the minibuffer after C-c C-I Inspect value (evaluated): I input (all-users) I can see the data but not edit it in the slime inspector, because the same condition is thrown as above. Can the slime inspector be customized in a way that it wraps calls to setf inside transactions? PS: if you run the above code, a directory named "current" will be created in your lisp's working directory. It saves the datastore on disk. Thank you, Christos Gitsis