DATAMONGO-897 - Fixed potential NullPointerException in QueryMapper.

If an association property points to an interface not containing the id property QueryMapper threw a NullPointerException in isAssociationConversionNecessary(…) as the lookup of the id property fails. 

We now check for the presence of an id property on the target type and check for assignability to indicated the need for conversion (usually in case when developers use raw ids in their update clauses, not the actual target instance.

Original pull request: #164.

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

@@ -250,14 +250,31 @@ public class QueryMapper {
 	 * type of the given value is compatible with the type of the given document field in order to deal with potential
 	 * query field exclusions, since MongoDB uses the {@code int} {@literal 0} as an indicator for an excluded field.
 	 * 
-	 * @param documentField
+	 * @param documentField must not be {@literal null}.
 	 * @param value
 	 * @return
 	 */
 	protected boolean isAssociationConversionNecessary(Field documentField, Object value) {
-		return documentField.isAssociation() && value != null
-				&& (documentField.getProperty().getActualType().isAssignableFrom(value.getClass()) //
-				|| documentField.getPropertyEntity().getIdProperty().getActualType().isAssignableFrom(value.getClass()));
+
+		Assert.notNull(documentField, "Document field must not be null!");
+
+		if (value == null) {
+			return false;
+		}
+
+		if (!documentField.isAssociation()) {
+			return false;
+		}
+
+		Class<? extends Object> type = value.getClass();
+		MongoPersistentProperty property = documentField.getProperty();
+
+		if (property.getActualType().isAssignableFrom(type)) {
+			return true;
+		}
+
+		MongoPersistentEntity<?> entity = documentField.getPropertyEntity();
+		return entity.hasIdProperty() && entity.getIdProperty().getActualType().isAssignableFrom(type);
 	}
 
 	/**

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

@@ -26,6 +26,7 @@ import java.util.List;
 
 import org.hamcrest.Matcher;
 import org.hamcrest.collection.IsIterableContainingInOrder;
+import org.hamcrest.core.IsEqual;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -37,6 +38,7 @@ import org.springframework.data.annotation.Id;
 import org.springframework.data.convert.WritingConverter;
 import org.springframework.data.mapping.model.MappingException;
 import org.springframework.data.mongodb.MongoDbFactory;
+import org.springframework.data.mongodb.core.DBObjectTestUtils;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 import org.springframework.data.mongodb.core.query.Update;
@@ -452,6 +454,81 @@ public class UpdateMapperUnitTests {
 
 	}
 
+	/**
+	 * @see DATAMONGO-897
+	 */
+	@Test
+	public void updateOnDbrefPropertyOfInterfaceTypeWithoutExplicitGetterForIdShouldBeMappedCorrectly() {
+
+		Update update = new Update().set("referencedDocument", new InterfaceDocumentDefinitionImpl("1", "Foo"));
+		DBObject mappedObject = mapper.getMappedObject(update.getUpdateObject(),
+				context.getPersistentEntity(DocumentWithReferenceToInterfaceImpl.class));
+
+		DBObject $set = DBObjectTestUtils.getAsDBObject(mappedObject, "$set");
+		Object model = $set.get("referencedDocument");
+
+		DBRef expectedDBRef = new DBRef(factory.getDb(), "interfaceDocumentDefinitionImpl", "1");
+		assertThat(model, allOf(instanceOf(DBRef.class), IsEqual.<Object> equalTo(expectedDBRef)));
+	}
+
+	@org.springframework.data.mongodb.core.mapping.Document(collection = "DocumentWithReferenceToInterface")
+	static interface DocumentWithReferenceToInterface {
+
+		String getId();
+
+		InterfaceDocumentDefinitionWithoutId getReferencedDocument();
+
+	}
+
+	static interface InterfaceDocumentDefinitionWithoutId {
+
+		String getValue();
+	}
+
+	static class InterfaceDocumentDefinitionImpl implements InterfaceDocumentDefinitionWithoutId {
+
+		@Id String id;
+		String value;
+
+		public InterfaceDocumentDefinitionImpl(String id, String value) {
+
+			this.id = id;
+			this.value = value;
+		}
+
+		@Override
+		public String getValue() {
+			return this.value;
+		}
+
+	}
+
+	static class DocumentWithReferenceToInterfaceImpl implements DocumentWithReferenceToInterface {
+
+		private @Id String id;
+
+		@org.springframework.data.mongodb.core.mapping.DBRef//
+		private InterfaceDocumentDefinitionWithoutId referencedDocument;
+
+		public String getId() {
+			return id;
+		}
+
+		public void setId(String id) {
+			this.id = id;
+		}
+
+		public void setModel(InterfaceDocumentDefinitionWithoutId referencedDocument) {
+			this.referencedDocument = referencedDocument;
+		}
+
+		@Override
+		public InterfaceDocumentDefinitionWithoutId getReferencedDocument() {
+			return this.referencedDocument;
+		}
+
+	}
+
 	static interface Model {}
 
 	static class ModelImpl implements Model {