@JsonBackReference
    @JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "client")
    private Set<Extension> ext = new HashSet<>();

    @JsonManagedReference
    @JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "id_client")
    private Client client;

127.0.0.1:8080/ext/client/1

[
    {
        "id": 1,
        "startDate": "2018-10-14T19:00:00.000+0000",
        "days": 30,
        "flag": 1,
        "hexSha": "3b07bb67dd55b7e38fa2316bbb50bae391a5063a6015091425ad18eb1b8aaa61192e7da7ffdc3a4c639580aa2d810e4bf5d60fd20815366f0aad6e6697fe6628614088fcс2",
        "status": 1,
        "client": {
            "id": 1,
            "name": "test",
            "email": "test@test.uz",
            "phone": "+998998998998",
            "hexc": "c84606c348332d91ee47080ea95bac9f",
            "chatId": null
        }
    }
]

127.0.0.1:8080/clients/client/1

{
    "id": 1,
    "name": "test",
    "email": "test@test.uz",
    "phone": "+998998998998",
    "hexc": "c84606c348332d91ee47080ea95bac9f",
    "chatId": null
}