How to map generic collections with Spring Data(MongoDB)?

Question

I have some problems with mapping my inner collection. Items can have different content types. Here is my Item class:

@Document(collection = "items")
public class Item{
@Id
private ObjectId id;   
private List<? super Content> content;
...
}

Content is a base class for different content for this Item.

public class YoutubeVideo implements Content{
     private String url;
}

public class Image implements Content{
     private String location;
}
...

After saving (saving finishing with no problems) Item with one Image and two YoutubeVideo classes in content collection i getting this JSON

{ "_id" : { "$oid" : "4e423dcf7f3a0d12265da46c"} 
"content" : [ 
{ "location" : "hdd path"} , { "url" : "url path"} , { "url" : "url path"}
]}

It is not this JSON I expected to see. And understandable why it is not possible to load and deserialize this document.

java.lang.RuntimeException: Can not map ? super trngl.mongo.domain.content.Content

How you would map this kind of object? I do not want to serialize and deserialize objects manualy. Is it possible?

Found interestion converters class: mapping-explicit-converters

Solution 1

It should be sufficient to use List<Content> as you can't access the concrete types from a mixed content list anyway. (Btw. super is definitely wrong here as your not storing supertyes of Content but subtypes. Extends on the other hand wouldn't add any additional value).

List<Content> should work with the latest snapshots for MongoTemplate as we fixed quite some bugs since the last milestone release. If you're using our repository abstraction make sure Content is an abstract class containing the id property. There is an open issue you might wanna watch for us to support interfaces as repository managed types as well.

Solution 2

Just as an FYI (since we hunted for this for a good morning after trying to figure out why we were getting our objects returned as LinkedHashMaps of the members), this issue also occurs (in 1.0.2.RELEASE), when you try to store a Collection<Content> in the form:

@Document(collection = "items")
public class Item{
@Id
private ObjectId id;   
private Collection<Content> content;
...
}

The solution, as above, is to switch it to a List<Content>.