[[mandatory-properties-in-subtypes]]
= Mandatory Properties in Subtypes

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If you have a hierarchy of classes then you need to decide which inheritance strategy to use.

* "table per hierarchy", or "rollup" (`InheritanceStrategy.SUPERCLASS_TABLE`) +
+
whereby a single table corresponds to the superclass, and also holds the properties of the subtype (or subtypes) being rolled up

* "table per class" (`InheritanceStrategy.NEW_TABLE`) +
+
whereby there is a table for both superclass and subclass, in 1:1 correspondence

* "rolldown" (`InheritanceStrategy.SUBCLASS_TABLE`) +
+
whereby a single table holds the properties of the subtype, and also holds the properties of its supertype

In the first "rollup" case, we can have a situation where - logically speaking - the property is mandatory in the subtype - but it must be mapped as nullable in the database because it is n/a for any other subtypes that are rolled up.

In this situation we must tell JDO that the column is optional, but to Apache Isis we want to enforce it being mandatory. This can be done using the `@Property(optionality=Optionality.MANDATORY)` annotation.

For example:

[source,java]
----
import javax.jdo.annotations.Column;
import javax.jdo.annotations.Inheritance;
import javax.jdo.annotations.InheritanceStrategy;
import lombok.Getter;
import lombok.Setter;

@Inheritance(strategy = InheritanceStrategy.SUPER_TABLE)
public class SomeSubtype extends SomeSuperType {
    @Column(allowsNull="true")
    @Property(optionality=Optionality.MANDATORY)
    @Getter @Setter
    private LocalDate date;
}
----


