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  package org.apache.cayenne.access;
  
  import java.util.List;
  
  import org.apache.cayenne.CayenneRuntimeException;
  import org.apache.cayenne.Fault;
  import org.apache.cayenne.ObjectContext;
  import org.apache.cayenne.PersistenceState;
  import org.apache.cayenne.Persistent;
  import org.apache.cayenne.query.RelationshipQuery;
  
  /**
   * @since 3.0
   */
  public class ToOneFault extends Fault {
  
      /**
       * Resolves this fault to a Persistent.
       */
      @Override
      public Object resolveFault(Persistent sourceObject, String relationshipName) {
  
          if (sourceObject.getObjectContext() == null) {
              throw new IllegalStateException(
                      "Null ObjectContext. Can't read a to-one relationship '"
                              + relationshipName
                              + "' for an object with ID: "
                              + sourceObject.getObjectId());
          }
  
          int state = sourceObject.getPersistenceState();
          if (state == PersistenceState.NEW) {
              return null;
          }
  
          Object target = doResolveFault(sourceObject, relationshipName);
  
          // must update the diff for the object
  
          ObjectContext context = sourceObject.getObjectContext();
          if ((state == PersistenceState.MODIFIED || state == PersistenceState.DELETED)
                  && context instanceof DataContext) {
  
              ObjectDiff diff = ((DataContext) context)
                      .getObjectStore()
                      .getChangesByObjectId()
                      .get(sourceObject.getObjectId());
  
              if (diff != null) {
                  diff.updateArcSnapshot(relationshipName, (Persistent) target);
              }
          }
  
          return target;
      }
  
      Object doResolveFault(Persistent sourceObject, String relationshipName) {
          RelationshipQuery query = new RelationshipQuery(
                  sourceObject.getObjectId(),
                  relationshipName,
                  false);
  
          List objects = sourceObject.getObjectContext().performQuery(query);
  
          if (objects.isEmpty()) {
              return null;
          } else if (objects.size() == 1) {
              return objects.get(0);
          } else {
              throw new CayenneRuntimeException("Error resolving to-one fault. "
                      + "More than one object found. Source Id: %s, relationship: %s"
                      , sourceObject.getObjectId(), relationshipName);
          }
      }
  }
  