DATAGRAPH-1423 - Add a better error message when persisting invalid implicit entities.

src/main/java/org/springframework/data/neo4j/repository/event/IdPopulator.java

@@ -48,8 +48,14 @@ final class IdPopulator {
 		Neo4jPersistentEntity<?> nodeDescription = neo4jMappingContext.getRequiredPersistentEntity(entity.getClass());
 		IdDescription idDescription = nodeDescription.getIdDescription();
 
-		if (idDescription == null && nodeDescription.isRelationshipPropertiesEntity()) {
-			return entity;
+		if (idDescription == null) {
+			if (nodeDescription.isRelationshipPropertiesEntity()) {
+				return entity;
+			} else {
+				throw new IllegalStateException(
+						"Cannot persist implicit entity due to missing id property on " + nodeDescription.getUnderlyingClass()
+								+ ".");
+			}
 		}
 
 		// Filter in two steps to avoid unnecessary object creation.

src/test/java/org/springframework/data/neo4j/repository/event/IdPopulatorTest.java

@@ -16,6 +16,7 @@
 package org.springframework.data.neo4j.repository.event;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
@@ -94,12 +95,24 @@ class IdPopulatorTest {
 		assertThat(populatedSample.theId).isEqualTo("Not necessary unique.");
 	}
 
+	@Test // DATAGRAPH-1423
+	void shouldNotFailWithNPEOnMissingIDGenerator() {
+
+		IdPopulator idPopulator = new IdPopulator(new Neo4jMappingContext());
+		assertThatIllegalStateException().isThrownBy(() -> idPopulator.populateIfNecessary(new ImplicitEntityWithoutId()))
+				.withMessage("Cannot persist implicit entity due to missing id property on " + ImplicitEntityWithoutId.class + ".");
+	}
+
 	@Node
 	static class Sample {
 
 		@Id @GeneratedValue(DummyIdGenerator.class) private String theId;
 	}
 
+	static class ImplicitEntityWithoutId {
+
+	}
+
 	static class DummyIdGenerator implements IdGenerator<String> {
 
 		@Override