Delete children in one to many relationship

Question

Having an issue with a pretty basic Hibernate 3.6.10 implementation.

I have two classes, schedule and event. The only way to get to an event is through schedule, so I've modeled as a one to many relationship with a schedule having many events.

Here is schedule:

/**
 * 
 */
package com.heavyweightsoftware.leal.model.schedule;

import java.util.Collection;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import com.heavyweightsoftware.leal.helper.DateHelper;
import com.heavyweightsoftware.leal.model.Pojo;

/**
 * A particular collection of events that can be shared
 * @author Thom
 */
@Entity
@Table(name = "SCHEDULE")
@NamedQueries( {
    @NamedQuery(name = Schedule.COUNT_SCHED_ID,      query =  "SELECT COUNT(*) " +
                                                              "FROM Schedule s " +
                                                              "WHERE s.scheduleId = :scheduleId"),
    @NamedQuery(name = Schedule.COUNT_SCHED_NAME,    query =  "SELECT COUNT(*) " +
                                                              "FROM   ScheduleRole r, Schedule s, SystemUser u " +
                                                              "WHERE  u.email = :email " +
                                                               " AND u.id = r.systemUserId " +
                                                               " AND r.scheduleId = s.id " +
                                                               " AND s.name = :scheduleName "),
    @NamedQuery(name = Schedule.QUERY_EVENTS_BY_USER, query =  "SELECT  r.roleType, s " +
                                                               "FROM    Schedule s, ScheduleRole r, SystemUser u " +
                                                               "WHERE   u.email = :email " +
                                                               "  AND   u.id = r.systemUserId " +
                                                               "  AND   r.scheduleId = s.id " +
                                                               " ")
    }
)
public class Schedule extends Pojo {

    public static final int        LENGTH_SCHEDULE_ID        = 32;
    public static final String     COUNT_SCHED_ID            = "countScheduleId";
    public static final String     COUNT_SCHED_NAME          = "countScheduleName";
    public static final String     QUERY_EVENTS_BY_USER      = "findEventsByUser";

    @Column(name = "ID", nullable=false)
    @Id @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer     id;

    @Column(name = "NAME", nullable=false)
    private String      name;

    @Column(name = "SCHEDULE_ID", nullable=false, unique=true, length=LENGTH_SCHEDULE_ID)
    private String      scheduleId;

    @OneToMany(cascade = CascadeType.ALL, fetch=FetchType.EAGER)
    private Collection<Event>   events;

    /* (non-Javadoc)
     * @see com.heavyweightsoftware.leal.model.Pojo#toString()
     */
    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(super.toString());
        sb.append('|');
        sb.append(getName());
        sb.append('|');
        sb.append(getScheduleId());
        return sb.toString();
    }

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = super.hashCode();
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        result = prime * result
                + ((scheduleId == null) ? 0 : scheduleId.hashCode());
        return result;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!super.equals(obj))
            return false;
        if (getClass() != obj.getClass())
            return false;
        Schedule other = (Schedule) obj;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (scheduleId == null) {
            if (other.scheduleId != null)
                return false;
        } else if (!scheduleId.equals(other.scheduleId))
            return false;
        return true;
    }

    /**
     * @return the id
     */
    public final Integer getId() {
        return id;
    }

    /**
     * @param id the id to set
     */
    public final void setId(Integer id) {
        this.id = id;
    }

    /**
     * @return the name
     */
    public final String getName() {
        return name;
    }

    /**
     * @param name the name to set
     */
    public final void setName(String name) {
        this.name = name;
    }

    /**
     * @return
     */
    public String getScheduleId() {
        return scheduleId == null?scheduleId = DateHelper.getUniqueID():scheduleId;
    }

    /**
     * @param scheduleId
     */
    public void setScheduleId(String scheduleId) {
        this.scheduleId = scheduleId;
    }

    /**
     * @return the events
     */
    public Collection<Event> getEvents() {
        return events;
    }

    /**
     * @param events the events to set
     */
    public void setEvents(Collection<Event> events) {
        this.events = events;
    }
}

And here is Event:

/**
 * 
 */
package com.heavyweightsoftware.leal.model.schedule;

import java.util.Calendar;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.NamedQueries;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

import com.heavyweightsoftware.leal.model.Pojo;

/**
 * A particular event entry in a calendar
 * @author Thom
 */
@Entity
@Table(name = "EVENT")
@NamedQueries( {
    }
)
public class Event extends Pojo {

    /**
     * Length of the randomly generated event ID
     */
    private static final int            LENGTH_EVENT_ID             = 32;

    @Column(name = "ID", nullable=false)
    @Id @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer     id;

    @Column(name = "START_TIME")
    @Temporal(TemporalType.TIMESTAMP)
    private Calendar    start;

    @Column(name = "END_TIME")
    @Temporal(TemporalType.TIMESTAMP)
    private Calendar    end;

    @Column(name = "EVENT_NAME", nullable=false)
    private String      eventName;

    @Column(name = "EVENT_ID", nullable=false, unique=true, length=LENGTH_EVENT_ID)
    private String      eventId;

    @ManyToOne(fetch=FetchType.EAGER)
    private Schedule    schedule;

    @Column(name = "LOCATION")
    private String      location;

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = super.hashCode();
        result = prime * result + ((end == null) ? 0 : end.hashCode());
        result = prime * result
                + ((eventName == null) ? 0 : eventName.hashCode());
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        result = prime * result
                + ((location == null) ? 0 : location.hashCode());
        result = prime * result
                + ((schedule == null) ? 0 : schedule.hashCode());
        result = prime * result + ((start == null) ? 0 : start.hashCode());
        return result;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (!super.equals(obj))
            return false;
        if (getClass() != obj.getClass())
            return false;
        Event other = (Event) obj;
        if (end == null) {
            if (other.end != null)
                return false;
        } else if (!end.equals(other.end))
            return false;
        if (eventName == null) {
            if (other.eventName != null)
                return false;
        } else if (!eventName.equals(other.eventName))
            return false;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (location == null) {
            if (other.location != null)
                return false;
        } else if (!location.equals(other.location))
            return false;
        if (schedule == null) {
            if (other.schedule != null)
                return false;
        } else if (!schedule.equals(other.schedule))
            return false;
        if (start == null) {
            if (other.start != null)
                return false;
        } else if (!start.equals(other.start))
            return false;
        return true;
    }

    /* (non-Javadoc)
     * @see com.heavyweightsoftware.leal.model.Pojo#toString()
     */
    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(super.toString());
        sb.append('|');
        sb.append(getEventName());
        sb.append('|');
        sb.append(getTimestamp(getStart()));
        sb.append('-');
        sb.append(getTimestamp(getEnd()));
        sb.append("|scheduleId=");
        sb.append(getSchedule().toString());
        sb.append('|');
        sb.append(getLocation());
        return sb.toString();
    }

    /**
     * @return the id
     */
    public final Integer getId() {
        return id;
    }

    /**
     * @param id the id to set
     */
    public final void setId(Integer id) {
        this.id = id;
    }

    /**
     * The start date of the event in UTC
     * @return the start
     */
    public final Calendar getStart() {
        return start;
    }

    /**
     * The start date of the event in UTC
     * @param start the start to set
     */
    public final void setStart(Calendar start) {
        this.start = start;
    }

    /**
     * The end date of the event in UTC
     * @return the end
     */
    public final Calendar getEnd() {
        return end;
    }

    /**
     * The end date of the event in UTC
     * @param end the end to set
     */
    public final void setEnd(Calendar end) {
        this.end = end;
    }

    /**
     * @return the eventId
     */
    public String getEventId() {
        return eventId;
    }

    /**
     * @param eventId the eventId to set
     */
    public void setEventId(String eventId) {
        this.eventId = eventId;
    }

    /**
     * @return the eventName
     */
    public final String getEventName() {
        return eventName;
    }

    /**
     * @param eventName the eventName to set
     */
    public final void setEventName(String eventName) {
        this.eventName = eventName;
    }

    /**
     * @return the location
     */
    public final String getLocation() {
        return location;
    }

    /**
     * @param location the location to set
     */
    public final void setLocation(String location) {
        this.location = location;
    }

    /**
     * @return the schedule
     */
    public Schedule getSchedule() {
        return schedule;
    }

    /**
     * @param schedule the schedule to set
     */
    public void setSchedule(Schedule schedule) {
        this.schedule = schedule;
    }
}

Everything is working fine except delete. When I implement my DAO through spring and try to delete my schedule, it throws an Integrity Constraint on the Event. It doesn't work if I try to delete the events first either.

getHibernateTemplate.delete(schedule);

How do I do this?

I have reviewed Hibernate Delete Cascade but I do not see how this applies in my situation.

Here is the delete method that I've written:

public void delete(Schedule sched) {
    //grab the events
    Collection<Event> events = sched.getEvents();
    //delete all children
getHibernateTemplate().deleteAll(events);
//flush the buffers
    getHibernateTemplate().flush();
    //clear the collection
    sched.getEvents().clear();
    //delete the schedule
    super.delete(sched);
}

Thanks.

Answer 1

After trial and error and research, here's where I ended up.

@OneToMany(cascade = CascadeType.ALL, fetch=FetchType.EAGER, orphanRemoval=true)
private Collection<Event>   events;

And then Event:

@ManyToOne(fetch=FetchType.EAGER)
private Schedule    schedule;

And then the delete method.

public void delete(Schedule sched) 
        throws DataAccessException{
    //grab the events
    Collection<Event> events = sched.getEvents();
    //clear the collection
    events.clear();
    //flush the buffers
    getHibernateTemplate().flush();
    //reload
    sched = retrieve(sched);
    //delete the schedule
    super.delete(sched);
}

Thanks for all the help!

Answer 2

In short, you're trying to cascade deletions from a parent entity to its children. This a very common problem.

This question offers the answers you seek: Hibernate Delete Cascade

Additionally, consider adding ON DELETE CASCADE to your database schema to make this behavior explicit in your DBMS.