Using generic objects with Spring Data Mongo

Question

I am using Spring Data Mongo to interface my program with an instance of MongoDB. I store inside Mongo a type similar to the following.

@Document
class A<T> {
    @Id String id;
    Instant createdAt;
    List<T> values;
}

As you can see, the generic type T is used in a property inside the main document. I have some problems in extracting such document using queries. I am currently using something similar to the following statement.

List<A> list = 
    mongoTemplate.find(Query.query(Criteria.where("id").in("id1", "id2"),
                       A.class,
                       "collectionName");

Unfortunately, the above code does not offer any support for generic fields. I looked at the documentation and at the code of MongoTemplate, but I did not find anything.

Some others templates classes of Spring offer this support. Take RestTemplate for example. There are many signatures of the exchange methods that use a ParameterizedTypeReference<T> to achieve something similar to what I am searching for MongoTemplate (this, for example).

In my opinion, it would be useful to have something similar also in MongoTemplate.

Is there a way to handle the generic type during the extraction process?

Thanks.

Answer 1

I don't think there is a way to support generic documents with Spring Data MongoDB.

As clearly explained by Oliver Gierke in his comment:

Without a subtype of A<T> that binds T to some type, there's no point in even using a generic type here. You could just stick to List<Object>.

The best way to achieve what you need is to creare a sub-type of A for every values-type. Something like this:

@Document
public class StringA extends A<String> { ... }

@Document
public class IntegerA extends A<Integer> { ... }