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By default GORM classes use `table-per-hierarchy` inheritance mapping. This has the disadvantage that columns cannot have a `NOT-NULL` constraint applied to them at the database level. If you would prefer to use a `table-per-subclass` inheritance strategy you can do so as follows:

[source,java]
----
class Payment {
    Integer amount

    static mapping = {
        tablePerHierarchy false
    }
}

class CreditCardPayment extends Payment {
    String cardNumber
}
----

The mapping of the root `Payment` class specifies that it will not be using `table-per-hierarchy` mapping for all child classes.