Introduced OR mapping for the Cargo->(DeliveryHistory)->HandlingEvent relation, instead of explicitly deciding whether or not to fetch handling events when loading a particular cargo.

dddsample/src/main/java/se/citerus/dddsample/domain/Cargo.java

@@ -36,7 +36,7 @@ public final class Cargo implements Entity<Cargo> {
   private Location origin;
   private Location destination;
   private Itinerary itinerary;
-  private DeliveryHistory deliveryHistory = DeliveryHistory.EMPTY_DELIVERY_HISTORY;
+  private DeliveryHistory deliveryHistory;
 
   /**
    * @param trackingId tracking id
@@ -44,7 +44,9 @@ public final class Cargo implements Entity<Cargo> {
    * @param destination destination location
    */
   public Cargo(final TrackingId trackingId, final Location origin, final Location destination) {
-    Validate.noNullElements(new Object[] {trackingId, origin, destination});
+    Validate.notNull(trackingId);
+    Validate.notNull(origin);
+    Validate.notNull(destination);
 
     this.trackingId = trackingId;
     this.origin = origin;
@@ -77,35 +79,31 @@ public final class Cargo implements Entity<Cargo> {
   }
 
   /**
-   * @return Final destination.
+   * @return Destination of the cargo.
    */
   public Location destination() {
     return this.destination;
   }
 
   /**
-   * @return Delivery history.
+   * @return The delivery history. Never null.
    */
   public DeliveryHistory deliveryHistory() {
-    return this.deliveryHistory;
+    return nullSafe(this.deliveryHistory, DeliveryHistory.EMPTY_DELIVERY_HISTORY);
   }
 
   /**
-   * @return The itinerary.
+   * @return The itinerary. Never null.
    */
   public Itinerary itinerary() {
-    if (this.itinerary == null) {
-      return Itinerary.EMPTY_ITINERARY;
-    } else {
-      return this.itinerary;
-    }
+    return nullSafe(this.itinerary, Itinerary.EMPTY_ITINERARY);
   }
 
   /**
    * @return Last known location of the cargo, or Location.UNKNOWN if the delivery history is empty.
    */
   public Location lastKnownLocation() {
-    final HandlingEvent lastEvent = deliveryHistory.lastEvent();
+    final HandlingEvent lastEvent = deliveryHistory().lastEvent();
     if (lastEvent != null) {
       return lastEvent.location();
     } else {
@@ -128,11 +126,11 @@ public final class Cargo implements Entity<Cargo> {
   public void attachItinerary(final Itinerary itinerary) {
     Validate.notNull(itinerary);
 
-    // Decouple the old itinerary from this cargo 
+    // Decouple the old itinerary from this cargo
     itinerary().setCargo(null);
     // Couple this cargo and the new itinerary
     this.itinerary = itinerary;
-    itinerary().setCargo(this);
+    this.itinerary.setCargo(this);
   }
 
   /**
@@ -146,7 +144,7 @@ public final class Cargo implements Entity<Cargo> {
   /**
    * @param deliveryHistory Cargo delivery history
    */
-  public void setDeliveryHistory(final DeliveryHistory deliveryHistory) {
+  void setDeliveryHistory(final DeliveryHistory deliveryHistory) {
     Validate.notNull(deliveryHistory);
     this.deliveryHistory = deliveryHistory;
   }
@@ -163,23 +161,23 @@ public final class Cargo implements Entity<Cargo> {
    * @return <code>true</code> if the cargo has been misdirected,
    */
   public boolean isMisdirected() {
-    final HandlingEvent lastEvent = deliveryHistory.lastEvent();
-    if (itinerary == null || lastEvent == null) {
+    final HandlingEvent lastEvent = deliveryHistory().lastEvent();
+    if (lastEvent == null) {
       return false;
     } else {
-      return !itinerary.isExpected(lastEvent);
+      return !itinerary().isExpected(lastEvent);
     }
   }
 
   /**
-   * Does not take into account the possibility of the cargo havin been
+   * Does not take into account the possibility of the cargo having been
    * (errouneously) loaded onto another carrier after it has been unloaded
    * at the final destination.
    *
    * @return True if the cargo has been unloaded at the final destination.
    */
   public boolean isUnloadedAtDestination() {
-    for (HandlingEvent event : deliveryHistory.eventsOrderedByCompletionTime()) {
+    for (HandlingEvent event : deliveryHistory().eventsOrderedByCompletionTime()) {
       if (HandlingEvent.Type.UNLOAD.equals(event.type())
         && destination.equals(event.location())) {
         return true;
@@ -214,6 +212,11 @@ public final class Cargo implements Entity<Cargo> {
     return trackingId.hashCode();
   }
 
+  // Utility for Null Object Pattern - should be moved out of this class
+  private <T> T nullSafe(T actual, T safe) {
+    return actual == null ? safe : actual;
+  }
+
   Cargo() {
     // Needed by Hibernate
   }

dddsample/src/main/java/se/citerus/dddsample/domain/DeliveryHistory.java

@@ -11,11 +11,11 @@ import java.util.*;
  */
 public final class DeliveryHistory implements ValueObject<DeliveryHistory> {
 
-  private final Set<HandlingEvent> events;
+  private Set<HandlingEvent> events;
 
   public static final DeliveryHistory EMPTY_DELIVERY_HISTORY = new DeliveryHistory(Collections.EMPTY_SET);
 
-  public DeliveryHistory(final Collection<HandlingEvent> events) {
+  DeliveryHistory(final Collection<HandlingEvent> events) {
     this.events = new HashSet<HandlingEvent>(events);
   }
 
@@ -98,4 +98,8 @@ public final class DeliveryHistory implements ValueObject<DeliveryHistory> {
     return events.hashCode();
   }
 
+  DeliveryHistory() {
+    // Needed by Hibernate
+  }
+
 }

dddsample/src/main/java/se/citerus/dddsample/repository/CargoRepositoryHibernate.java

@@ -2,7 +2,6 @@ package se.citerus.dddsample.repository;
 
 import org.springframework.stereotype.Repository;
 import se.citerus.dddsample.domain.Cargo;
-import se.citerus.dddsample.domain.DeliveryHistory;
 import se.citerus.dddsample.domain.Itinerary;
 import se.citerus.dddsample.domain.TrackingId;
 
@@ -18,41 +17,21 @@ public class CargoRepositoryHibernate extends HibernateRepository implements Car
   HandlingEventRepository handlingEventRepository;
 
   public Cargo find(TrackingId tid) {
-    Cargo cargo = (Cargo) getSession().
-            createQuery("from Cargo where trackingId = :tid").
+    // Query for id and then perform a standard load()
+    // to use metadata-defined query and lazy proxy access
+    Long id = (Long) getSession().
+            createQuery("select id from Cargo where trackingId = :tid").
             setParameter("tid", tid).
             uniqueResult();
-    if (cargo == null) {
-      return null;
-    }
-    /*  There's no OR-mapped relation between the cargo delivery history and its handling events
-        because the handling events are in a different aggregate.
-
-        TODO: investigate the following scenario to motivate this construction 
-        If we had to use pessimistic locking on cargo,
-        it might be cumbersome to map the relation between cargo and handling event,
-        since we want to be able to insert handling events regardless of locking status on cargo.
-
-
-        If this extra database call were a problem, you might want to use a different model.
-        For example, you could calculate the effect/status of the cargo and store it separate from the
-        handling events. */
-
-    /*
-        TODO:
-        the decision whether or not to include the delivery history when loading cargo
-        seems to belong in the service layer, which defines use cases.
-     */
-    DeliveryHistory deliveryHistory = new DeliveryHistory(handlingEventRepository.findEventsForCargo(tid));
-    cargo.setDeliveryHistory(deliveryHistory);
 
-    return cargo;
+    return (Cargo) getSession().load(Cargo.class, id);
   }
 
   public void save(Cargo cargo) {
     getSession().persist(cargo);
 
-    // Delete orphaned itineraries
+    // Delete orphaned itineraries - conceptually the responsibility
+    // of the Cargo aggregate
     final List<Itinerary> orphans = getSession().
       createQuery("from Itinerary where cargo = null").
       list();

dddsample/src/main/resources/se/citerus/dddsample/domain/Cargo.hbm.xml

@@ -12,6 +12,12 @@
     <component name="trackingId" unique="true" update="false">
       <property name="id" column="tracking_id"/>
     </component>
+    <component name="deliveryHistory" lazy="true" update="false">
+      <set name="events" lazy="true" cascade="none">
+        <key column="cargo_id"/>
+        <one-to-many class="se.citerus.dddsample.domain.HandlingEvent"/>
+      </set>
+    </component>
     <many-to-one name="origin" column="origin_id" cascade="none" fetch="join" update="false" foreign-key="origin_fk"/>
     <many-to-one name="destination" column="destination_id" cascade="none" fetch="join" foreign-key="destination_fk"/>
     <one-to-one name="itinerary" property-ref="cargo" cascade="all" fetch="join"/>