问题
If mapping inheritance using table per concrete class, while mapping concrete classes simply just as any other class (without union-subclass), NHibernate allows same PKs across subclasses. For example if you have BillingDetails and subclasses CreditCard and BankAccount, requesting all BillingDetails will get you all records from both tables, which can in turn have duplicate primary keys, which can be problematic due to not valid business identity of those objects. This of course is not the case with guids and such, but what about cases of plain identity or sequence id generators?
Simply, the question is, is there any real use of this scenarion of duplicate id keys when requesting polymorphic query like that? Could these duplicate ids make problems for NHibernate when handling entities internally?
回答1:
Implicit polymorphism is necessary when mapping legacy models where the tables do not share a common key.
There are no problems with getting a list of BillingDetails with CreditCard and BankAccount instances sharing the same Id, because NH knows they are unrelated (and they are treated as related only for the purposes of querying, by running one query for each mapped subclass)
You should not use it for greenfield development.
来源：https://stackoverflow.com/questions/9097933/any-real-use-for-table-per-concrete-class-with-implicit-polymorphism