Mock loading of navigation properties

This is a very simplified version of my code, that illustrates the specific problem.

Is there any way that I can control what happens when accountProductRepository.refresh() is called from the test?

Somehow I need to set the ProductPojo on the AccountProductPojo created in the buyProduct() method, so I don't get a null pointer when accessing the getProduct().getName() property.

refresh uses javax.persistence.EntityManager.refresh() to load the navigation properties based on the id's set in the buyProduct() method.

public class ProductServiceTest {
    @InjectMocks
    IProductService productService = new ProductService();
    @Mock
    IWriteANoteService writeANoteService;
    @Mock
    IAccountProductRepository accountProductRepository;

    @Test
    public void buyProductTest() {
        productService.buyProduct(1l, 1l);
    }
}

@Service
public class ProductService implements IProductService {
    @Autowired
    IWriteANoteService writeANoteService;

    @Autowired
    IAccountProductRepository accountProductRepository:

    public void buyProduct(Long productId, Long accountId) {
        AccountProductPojo accountProduct = new AccountProductPojo();
        accountProduct.setProductId(accountId);
        accountProduct.setAccountId(accountId);

        accountProductRepository.persist(accountProduct);
        // load navigation properties
        accountProductRepository.refresh(accountProduct);

        writeANoteService.writeAccountNote(accountId, "Bought product " + accountProduct.getProduct().getName());
    }
}

@Entity
@Table(name = "account_product")
public class AccountProductPojo {
    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "account_id")
    private Long accountId;

    @Column(name = "product_id")
    private Integer productId;

    @ManyToOne
    @JoinColumn(name = "product_id", insertable = false, updatable = false)
    private ProductPojo product;

    @OneToOne(fetch = FetchType.LAZY, targetEntity = AccountPojo.class)
    @JoinColumn(name = "account_id", insertable = false, updatable = false)
    private AccountPojo account;

    // getters and setters
}