We need random access to all objects at the time we use them (including all dependent objects). We also need objects to be updated on disk at the right moment.
MOP (MOOSE Object Persistancy) has been developed to perform this task.
When fetching an persistant element it may need to be read in from disk or it may already be present. When an object is dragged in it brings with it all those objects directly referenced including persistent collections (but not the contents of these collections). This means that the user is not normally aware of reading data.