Add a fix and a test for issue #19.

Since event subject traversal didn't filter its nodes for valid
types, deletion of a subject would result in the last event link
node being put in the result set, which caused a crash when we tried
to serialize it.

ehri-frames/src/main/java/eu/ehri/project/models/events/SystemEvent.java

@@ -73,8 +73,7 @@ public Iterable<AccessibleEntity> getSubjects() {
                     .loop("n", JavaHandlerUtils.noopLoopFunc, new PipeFunction<LoopPipe.LoopBundle<Vertex>, Boolean>() {
                         @Override
                         public Boolean compute(LoopPipe.LoopBundle<Vertex> vertexLoopBundle) {
-                            return !vertexLoopBundle.getObject().getVertices(Direction.IN,
-                                    Ontology.ENTITY_HAS_LIFECYCLE_EVENT).iterator().hasNext();
+                            return isValidTarget(vertexLoopBundle.getObject());
                         }
                     }));
         }
@@ -88,9 +87,7 @@ public AccessibleEntity getFirstSubject() {
                     .loop("n", JavaHandlerUtils.noopLoopFunc, new PipeFunction<LoopPipe.LoopBundle<Vertex>, Boolean>() {
                         @Override
                         public Boolean compute(LoopPipe.LoopBundle<Vertex> vertexLoopBundle) {
-                            return (!vertexLoopBundle.getObject().getVertices(Direction.IN,
-                                    Ontology.ENTITY_HAS_LIFECYCLE_EVENT).iterator().hasNext())
-                                    && vertexLoopBundle.getObject().getProperty(EntityType.TYPE_KEY) != null;
+                            return isValidTarget(vertexLoopBundle.getObject());
                         }
                     });
             return (AccessibleEntity)(subjects.iterator().hasNext()
@@ -109,5 +106,11 @@ public Boolean compute(LoopPipe.LoopBundle<Vertex> vertexLoopBundle) {
                         }
                     }));
         }
+
+        private boolean isValidTarget(Vertex vertex) {
+            return (!vertex.getVertices(Direction.IN,
+                    Ontology.ENTITY_HAS_LIFECYCLE_EVENT).iterator().hasNext())
+                    && vertex.getProperty(EntityType.TYPE_KEY) != null;
+        }
     }
 }

ehri-frames/src/test/java/eu/ehri/project/models/events/SystemEventTest.java

@@ -0,0 +1,45 @@
+package eu.ehri.project.models.events;
+
+import eu.ehri.project.definitions.EventTypes;
+import eu.ehri.project.models.UserProfile;
+import eu.ehri.project.models.base.Actioner;
+import eu.ehri.project.persistence.ActionManager;
+import eu.ehri.project.persistence.Bundle;
+import eu.ehri.project.persistence.BundleDAO;
+import eu.ehri.project.persistence.Serializer;
+import eu.ehri.project.test.AbstractFixtureTest;
+import eu.ehri.project.test.TestData;
+import org.junit.Test;
+import org.neo4j.helpers.collection.Iterables;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * @author Mike Bryant (http://github.com/mikesname)
+ */
+public class SystemEventTest extends AbstractFixtureTest {
+    @Test
+    public void testGetSubjects() throws Exception {
+        ActionManager am = new ActionManager(graph);
+        BundleDAO dao = new BundleDAO(graph);
+        Serializer serializer = new Serializer(graph);
+        // Create a user and log it
+        Bundle userBundle = Bundle.fromData(TestData.getTestUserBundle());
+        UserProfile user = dao.create(userBundle, UserProfile.class);
+
+        SystemEvent first = am.logEvent(user,
+                graph.frame(validUser.asVertex(), Actioner.class),
+                EventTypes.creation).getSystemEvent();
+        assertEquals(1, Iterables.count(first.getSubjects()));
+
+        // Delete the user and log it
+        SystemEvent second = am.logEvent(user,
+                graph.frame(validUser.asVertex(), Actioner.class),
+                EventTypes.deletion).getSystemEvent();
+        dao.delete(serializer.vertexFrameToBundle(user));
+
+        // First event should now have 0 subjects, since it's
+        // been deleted.
+        assertEquals(0, Iterables.count(first.getSubjects()));
+    }
+}