Database contains embedded objects, but MongoRepository returns empty List

Question

@Document("scenarios")
public class Scenario {
    @Id
    private String id;
    @Indexed(unique = true)
    private String name;
    private String description;
    @JsonSerialize(using = PlatformSerializer.class)
    private final List<Platform> platforms;
// getters and setters not shown
}

public class Platform {
    private final PlatformData platformData;
    @JsonSerialize(using = RowSerializer.class)
    private final List<Row> rows;
// getters and setters not shown
}

@Repository
public interface ScenarioRepository extends MongoRepository<Scenario, String> {
    Scenario findByName(String name);
}

The scenarios collection in the database contains embedded Platform objects in each document. And the Platform objects contain embedded Row objects. I can successfully get the Scenario object from the database using the findByName method in ScenarioRepository. The name and description values are correct. However, the platforms List is empty. I seem to recall that containers require an annotation in the POJO. Though the Scenario object was correctly saved to the database using this same code. What am I missing? Thanks!

JPShag · Accepted Answer · 2025-04-16 20:15:14Z

0

The cause is:
platforms (and rows) are declared final. When Spring‑Data reads the document it:

calls the no‑arg constructor (which leaves the list empty), then
tries to set each field via reflection – but it skips final fields, so the list stays empty.

Saving still works, because writing only serializes the existing object graph.

// Make the field non‑final 
private List<Platform> platforms

No extra Mongo annotations are needed for an embedded list.

answered Apr 16 at 20:15

JPShag

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