Techniques are described for expressing a constraint in a host language. A constraint can be defined in the host language as an event handler. The event handler can be registered to a built-in event associated with an object of the database that the constraint is intended to validate. When the object triggers the built-in event, the event handler is called to execute the constraint. Exemplary built-in events include onSave (trigger the event when the object is saved), onValidate (trigger the event when an explicit call to validate the object is detected), onInsert (trigger the event when the object is inserted), onUpdate (trigger the event when the object is updated), and onCheckBeforeSave (trigger the event when the object is checked prior to saving).