DATAMONGO-987 - Avoid creation of lazy-loading proxies for null-values.

We now avoid creating a lazy-loading proxy if we detect that the property-value in the backing DbObject for a @Lazy(true) annotated field is null.

Original pull request: #207.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -265,6 +265,11 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				MongoPersistentProperty property = association.getInverse();
 
 				Object value = dbo.get(property.getName());
+
+				if (value == null) {
+					return;
+				}
+
 				DBRef dbref = value instanceof DBRef ? (DBRef) value : null;
 				Object obj = dbRefResolver.resolveDbRef(property, dbref, new DbRefResolverCallback() {
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/DbRefMappingMongoConverterUnitTests.java

@@ -429,6 +429,27 @@ public class DbRefMappingMongoConverterUnitTests {
 		assertProxyIsResolved(result.dbRefEqualsAndHashcodeObjectMethodOverride2, true);
 	}
 
+	/**
+	 * @see DATAMONGO-987
+	 */
+	@Test
+	public void shouldNotGenerateLazyLoadingProxyForNullValues() {
+
+		DBObject dbo = new BasicDBObject();
+		ClassWithLazyDbRefs lazyDbRefs = new ClassWithLazyDbRefs();
+		lazyDbRefs.id = "42";
+		converter.write(lazyDbRefs, dbo);
+
+		ClassWithLazyDbRefs result = converter.read(ClassWithLazyDbRefs.class, dbo);
+
+		assertThat(result.id, is(lazyDbRefs.id));
+		assertThat(result.dbRefToInterface, is(nullValue()));
+		assertThat(result.dbRefToConcreteCollection, is(nullValue()));
+		assertThat(result.dbRefToConcreteType, is(nullValue()));
+		assertThat(result.dbRefToConcreteTypeWithPersistenceConstructor, is(nullValue()));
+		assertThat(result.dbRefToConcreteTypeWithPersistenceConstructorWithoutDefaultConstructor, is(nullValue()));
+	}
+
 	private Object transport(Object result) {
 		return SerializationUtils.deserialize(SerializationUtils.serialize(result));
 	}