@Entity
@IdClass(SamplingsPK.class)
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Samplings {

    @Id
    private int year;

    @Id
    @GeneratedValue
    private Integer sequenceId;

    @OneToMany(mappedBy = "sampling", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Samples> samples = new ArrayList<>();

}

@Entity
@IdClass(SamplesPK.class)
public class Samples extends BaseEntity { 

    @Id
    private String sampleLetter;

    @Id
    @ManyToOne(optional = false)
    @JoinColumns({
        @JoinColumn(name = "sampling_id", referencedColumnName = "sequenceId"),
        @JoinColumn(name = "sampling_year", referencedColumnName = "year")})
    private Samplings sampling;

  @OneToOne(mappedBy = "sample", cascade = CascadeType.ALL, fetch = FetchType.LAZY, orphanRemoval = true)
    private TestSamples testSamples;
}

select s from Samples s Join Fetch s.testSamples Join fetch s.sampling sp Left Join fetch sp.machine m Join fetch sp.product p Join  fetch p.productType pt