Dave Taubler Dave Taubler - 19 days ago 5
Java Question

Spring Data, Mongo, and @TypeAlias: reading not working

The issue

Awhile back I started using MongoDB and Spring Data. I'd left most of the default functionality in place, and so all of my documents were stored in MongoDB with a

_class
field pointing to the entity's fully-qualified class name.

Right away that didn't "smell" right to me, but I left it alone. Until recently, when I refactored a bunch of code, and suddenly none of my documents could be read back from MongoDB and converted into their (refactored/renamed) Java entities. I quickly realized that it was because there was now a fully-qualified-classname mismatch. I also quickly realized that--given that I might refactor again sometime in the future--if I didn't want all of my data to become unusable I'd need to figure something else out.

What I've tried

So that's what I'm doing, but I've hit a wall. I think that I need to do the following:


  • Annotate each entity with
    @TypeAlias("ta")
    where "ta" is a unique, stable string.

  • Configure and use a different
    TypeInformationMapper
    for Spring Data to use when converting my documents back into their Java entities; it needs to know, for example, that a type-alias of "widget.foo" refers to
    com.myapp.document.FooWidget
    .



I determined that I should use a
TypeInformationMapper
of type
org.springframework.data.convert.MappingContextTypeInformationMapper
. Supposedly a MappingContextTypeInformationMapper will scan my entities/documents to find @TypeAlias'ed documents and store an alias->to->class mapping. But I can't pass that to my MappingMongoConverter; I have to pass a subtype of MongoTypeMapper. So I am configuring a
DefaultMongoTypeMapper
, and passing a List of one
MappingContextTypeInformationMapper
as its "mappers" constructor arg.

Code

Here's the relevant part of my spring XML config:

<bean id="mongoTypeMapper" class="org.springframework.data.mongodb.core.convert.DefaultMongoTypeMapper">
<constructor-arg name="typeKey" value="_class"></constructor-arg>
<constructor-arg name="mappers">
<list>
<ref bean="mappingContextTypeMapper" />
</list>
</constructor-arg>
</bean>

<bean id="mappingContextTypeMapper" class="org.springframework.data.convert.MappingContextTypeInformationMapper">
<constructor-arg ref="mappingContext" />
</bean>

<bean id="mappingMongoConverter"
class="org.springframework.data.mongodb.core.convert.MappingMongoConverter">
<constructor-arg ref="mongoDbFactory" />
<constructor-arg ref="mappingContext" />
<property name="mapKeyDotReplacement" value="__dot__" />
<property name="typeMapper" ref="mongoTypeMapper"/>
</bean>

<bean id="mongoTemplate" class="org.springframework.data.mongodb.core.MongoTemplate">
<constructor-arg ref="mongoDbFactory" />
<constructor-arg ref="mappingMongoConverter" />
</bean>


Here's a sample entity/document:

@Document(collection="widget")
@TypeAlias("widget.foo")
public class FooWidget extends Widget {

// ...

}


One important note is that any such "Widget" entity is stored as a nested document in Mongo. So in reality you won't really find a populated "Widget" collection in my MongoDB instance. Instead, a higher-level "Page" class can contain multiple "widgets" like so:

@Document(collection="page")
@TypeAlias("page")
public class Page extends BaseDocument {

// ...

private List<Widget> widgets = new ArrayList<Widget>();

}


The error I'm stuck on

What happens is that I can save a Page along with a number of nested Widgets in Mongo. But when I try to read said Page back out, I get something like the following:

org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.myapp.document.Widget]: Is it an abstract class?


I can indeed see pages in Mongo containing
"_class" : "page"
, with nested widgets also containing
"_class" : "widget.foo"
It just appears like the mapping is not being applied in the reverse.

Is there anything I might be missing?

Answer

I spent a bunch of time with my debugger and the Spring Data source code, and I learned that Spring Data isn't as good as it probably should be with polymorphism as it should be, especially given the schema-less nature of NoSQL solutions like MongoDB. But ultimately what I did was to write my own type mapper, and that wasn't too tough.

The main problem was that, when reading in my Page document, the default mappers used by Spring Data would see a collection called widgets, then consult the Page class to determine that widgets pointed to a List, then consult the Widget class to look for @TypeAlias information. What I needed instead was a mapper that scanned my persistent entities up front and stored an alias-to-class mapping for later use. That's what my custom type mapper does.

I wrote a blog post discussing the details.