@OneToMany(mappedBy = "client", cascade = CascadeType.ALL)      // FetchType.LAZY по-умолчанию
private Set<Order> orders;
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@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "client_id")     // Имя fk
private Client client;
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Long foreignKey = order.getClient().getClientId();