I have some experience with OOP and RDBs, but very little exposure to web programming. I am trying to understand what non-trivial types of problems are solved by ORM. Of course, I am familiar with the need for data persistence, but I have never encountered a need for persisting relationships between objects, a situation which is indicated in many online articles about ORM.
I am not asking about the process of persisting a POJO to a database and restoring it later. Nor am I asking about why ORM frameworks are useful -- or a pain in the butt -- for doing so. I am particularly interested in how the need arises to persist and restore relationships between objects.
In various documentation, I have seen many examples of persisting POJOs to a database, but the examples have all been for only very simple objects that are essentially nothing more than records anyway: a constructor, some private fields, and getter/setter methods. The motivation for persisting such an "object-record" seems obvious and trivial. This example:
offers such an example, but goes on to discuss mismatch issues of granularity, inheritance, identity, associations, and navigation that are not motivated by the example.
If someone could offer a toy example of an instance where, say, the need arises to persist an inheritance relationship, I would be grateful. This might be blindingly obvious for anyone who has already encountered this situation but I have not and a great deal of searching and reading have not turned up any examples.