Objects can validate themselves and object databases store the class as well as the instances. This allows a database to contain a mix of different versions of

Objects can validate themselves and object databases store the class as well as the instances. This allows a database to contain a mix of different versions of a particular class of objects. The invoice from yesterday might have 5 fields and validate under one set of rules while todays invoice has 7 fields and validates itself under a completely different set of rules. Such polymorphism is trivial to implement with objects and very painful to implement withing a relational database. It's not a matter of whether a RDBMS can model something, it's a matter of how much effort is required.