EDIT

import org.apache.log4j.Logger;

import org.hibernate.annotations.Cascade;

import org.hibernate.annotations.CascadeType;

import org.hibernate.envers.Audited;

import org.hibernate.search.annotations.ContainedIn;

import org.hibernate.search.annotations.Indexed;

    @ManyToOne(fetch = FetchType.LAZY)

    @Cascade({CascadeType.SAVE_UPDATE})

//	TODO @NotNull // avoids creating a UNIQUE key for this field

    private Classification classification;

    @XmlElementWrapper(name = "childNodes")

    @XmlIDREF

    @XmlSchemaType(name = "IDREF")

    @OneToMany(mappedBy="parent", fetch=FetchType.LAZY)

    private Set<TaxonNode> childNodes = new HashSet<TaxonNode>();

    @XmlElement(name = "reference")