Content.java

package com.example;

import static javax.persistence.CascadeType.ALL;
import static javax.persistence.FetchType.EAGER;
import static javax.persistence.GenerationType.SEQUENCE;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.SequenceGenerator;
import javax.persistence.Transient;
import javax.persistence.Version;
import javax.validation.constraints.AssertTrue;

import org.apache.commons.lang.StringUtils;

@Entity
public class Content implements Serializable {
	
	private static final long serialVersionUID = 1L;

	/* Members ------------------------------------------------------------- */

	@Id @GeneratedValue(strategy = SEQUENCE, generator = "content_seq")
	@SequenceGenerator(name = "content_seq", sequenceName = "content_seq", initialValue = 1, allocationSize = 1)
	private Long id; 

	@OneToOne(cascade = ALL) @JoinColumn(name = "current_content_revision_id")
	private ContentRevision current;

	@OneToMany(mappedBy = "content", cascade = ALL, orphanRemoval = true, fetch = EAGER)
	private List<ContentRevision> revisionList;

	@Version
	private int version;


	/* Properties ---------------------------------------------------------- */

	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public ContentRevision getCurrent() {
		return current;
	}

	public void setCurrent(ContentRevision current) {
		this.current = current;
	}

	/**
	 * Checks whether the current revision is in the list of revisions.
	 * @return {@code true} if valid, {@code false} otherwise
	 */
	@Transient // ignored in JPA
	@AssertTrue // javax.validation
	public boolean isCurrentRevisionInList() {
		return current != null && getRevisionList().contains(current);
	}

	public List<ContentRevision> getRevisionList() {
		if (revisionList == null)
			revisionList = new ArrayList<ContentRevision>();
		return revisionList;
	}

	public void setRevisionList(List<ContentRevision> revisionList) {
		this.revisionList = revisionList;
	}


	/* Methods ------------------------------------------------------------- */

	@Override
	public int hashCode() {
		// using "id", since we don't have any other identifying property
		return id == null ? 0 : id.intValue();
	}

	@Override
	public boolean equals(Object obj) {
		if (obj == this)
			return true;
		else if (!(obj instanceof Content))
			return false;
		
		// compare using "id"
		Content that = (Content) obj;
		if (this.id == null || that.id == null)
			return false;
		else
			return this.id.equals(that.id);
	}

	/**
	 * Adds the given revision.
	 * @param revision revision to add
	 * @return {@code true} if added, {@code false} otherwise
	 */
	public boolean addRevision(ContentRevision revision) {
		if (revision != null) {
			revision.setContent(this);
			return getRevisionList().add(revision);
		}
		return false;
	}

	/**
	 * Adds a new revision with the given text.
	 * @param text text data
	 * @return the new revision, or {@code null} if text was empty
	 */
	public ContentRevision addRevision(String text) {
		if (!StringUtils.isBlank(text)) {
			ContentRevision revision = new ContentRevision(text);
			if (addRevision(revision))
				return revision;
		}
		return null;
	}

	/**
	 * Factory method to create a content with an initial revision.
	 * @param text text data
	 * @return the new content, or {@code null} if text was empty
	 */
	public static Content create(String text) {
		Content content = new Content();
		ContentRevision initial = content.addRevision(text);
		if (initial != null) {
			content.setCurrent(initial);
			return content;
		}
		return null;
	}

}

ContentRevision.java

package com.example;

import static javax.persistence.GenerationType.SEQUENCE;

import java.io.Serializable;
import java.util.Date;

import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.SequenceGenerator;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.persistence.Version;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;

import org.apache.commons.lang.StringUtils;
import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;

@Entity
public class ContentRevision implements Serializable {
	
	private static final long serialVersionUID = 1L;

	/* Members ------------------------------------------------------------- */

	@Id @GeneratedValue(strategy = SEQUENCE, generator = "content_revision_seq")
	@SequenceGenerator(name = "content_revision_seq", sequenceName = "content_revision_seq", initialValue = 1, allocationSize = 1)
	private Long id;

	@ManyToOne @JoinColumn(name = "content_id", updatable = false)
	private Content content;

	@Basic
	private String textData;

	@Temporal(TemporalType.TIMESTAMP) @Column(name = "reg_date")
	private Date registrationDate;

	@Version
	private int version;


	/* Constructors -------------------------------------------------------- */

	public ContentRevision(String text) {
		this.textData = StringUtils.trimToNull(text);
		this.registrationDate = new Date();
	}

	public ContentRevision() {
		this(null);
	}


	/* Properties ---------------------------------------------------------- */

	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	@NotNull
	public Content getContent() {
		return content;
	}

	public void setContent(Content content) {
		this.content = content;
	}

	@NotNull
	public String getTextData() {
		return textData;
	}

	public void setTextData(String textData) {
		this.textData = StringUtils.trimToNull(textData);
	}

	public Date getRegistrationDate() {
		return registrationDate;
	}

	public void setRegistrationDate(Date registrationDate) {
		this.registrationDate = registrationDate;
	}

	@Min(0)
	public int getVersion() {
		return version;
	}

	public void setVersion(int version) {
		this.version = version;
	}


	/* Methods ------------------------------------------------------------- */

	@Override
	public int hashCode() {
		return new HashCodeBuilder()
			.append(content)
			.append(textData)
			.append(registrationDate)
			.toHashCode();
	}

	@Override
	public boolean equals(Object obj) {
		if (obj == this)
			return true;
		else if (!(obj instanceof ContentRevision))
			return false;
		
		ContentRevision that = (ContentRevision) obj;
		return new EqualsBuilder()
			.append(this.content, that.content)
			.append(this.textData, that.textData)
			.append(this.registrationDate, that.registrationDate)
			.isEquals();
	}

}