Currently we have a validation that checks that all fields of an @IdClass are included in the corresponding entity class as @Id annotated properties.

We do not, however, check if there are more @Id annotated fields in the entity than what we expect, and this causes unexpected behavior (i.e. the additional @Id field might “override” the @IdClass and be set as simple-valued identifier for the entity class).

For example:

public static class PK {
  private Long id;
  
  private String username;
}

@Entity
@IdClass( PK.class ) 
public static class User {
  @Id
  private Long id;
  
  @Id
  private String username;
  
  @Id
  private Integer anotherId;
}

This should be considered a wrong mapping, and an appropriate error should be thrown.