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  package org.apache.cayenne.lifecycle.relationship;
  
  import org.apache.cayenne.Persistent;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * A faulting strategy that does batch-faulting of related objects whenever a first
   * ObjectId relationship is accessed.
   * 
   * @since 3.1
   */
  public class ObjectIdRelationshipBatchFaultingStrategy implements ObjectIdRelationshipFaultingStrategy {
  
      private ThreadLocal<List<ObjectIdBatchSourceItem>> batchSources;
  
      public ObjectIdRelationshipBatchFaultingStrategy() {
          this.batchSources = new ThreadLocal<>();
      }
  
      public void afterObjectLoaded(Persistent object) {
  
          String uuidProperty = objectIdPropertyName(object);
          String uuidRelationship = objectIdRelationshipName(uuidProperty);
          String uuid = (String) object.readProperty(uuidProperty);
          if (uuid == null) {
              object.writePropertyDirectly(uuidRelationship, null);
          }
          else {
              List<ObjectIdBatchSourceItem> sources = batchSources.get();
  
              if (sources == null) {
                  sources = new ArrayList<>();
                  batchSources.set(sources);
              }
  
              sources.add(new ObjectIdBatchSourceItem(object, uuid, uuidRelationship));
          }
      }
  
      public void afterQuery() {
  
          List<ObjectIdBatchSourceItem> sources = batchSources.get();
          if (sources != null) {
              batchSources.set(null);
  
              ObjectIdBatchFault batchFault = new ObjectIdBatchFault(sources
                      .get(0)
                      .getObject()
                      .getObjectContext(), sources);
  
              for (ObjectIdBatchSourceItem source : sources) {
                  source.getObject().writePropertyDirectly(
                          source.getObjectIdRelationship(),
                          new ObjectIdFault(batchFault, source.getId()));
              }
          }
      }
  
      String objectIdRelationshipName(String uuidPropertyName) {
          return "cay:related:" + uuidPropertyName;
      }
  
      String objectIdPropertyName(Persistent object) {
  
          ObjectIdRelationship annotation = object.getClass().getAnnotation(
                  ObjectIdRelationship.class);
  
          if (annotation == null) {
              throw new IllegalArgumentException(
                      "Object class is not annotated with @UuidRelationship: "
                              + object.getClass().getName());
          }
  
          // TODO: I guess we'll need to cache this metadata for performance if we are to
          // support inheritance lookups, etc.
  
          return annotation.value();
      }
  
  }
  