The subsequent entity has to attributes "parent" and "children":
both are of primitive type.
"children" is an element collection referencing "parent"
"parent" is nullable
However, the subsequent example below does not work anymore in Hibernate 5.0.4 and 5.1.0. It used to work fine in 4.3. For some reason, it expects parent to be not-nullable. Checking the metadata (org.hibernate.mapping.Property) shows that value is of Type SimpleValue and in there is a column "parent" that is not nullable anymore. At some point a nullable "parent" column is created, but it seems to get overriden by the element table of "children".