Jackson serialization issue. Only first object of the same entity serializes well

I develop a REST voting system where users can vote on restaurants. I have a Vote class which contains User, Restaurant and Date.

public class Vote extends AbstractBaseEntity {

    @NotNull
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    @NotNull
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "restaurant_id")
    private Restaurant restaurant;

    @Column(name = "date", nullable = false)
    @NotNull
    private LocalDate date;

}

I need to find all votes of the day. And if there are several votes for one restaurant, only first object serializes well. The other ones shows restaurant ID instead of Restaurant object as shown below:

[
    {
        "id": 100019,
        "user": null,
        "restaurant": {
            "id": 100004,
            "name": "KFC"
        },
        "date": "2020-08-28"
    },
    {
        "id": 100020,
        "user": null,
        "restaurant": 100004,
        "date": "2020-08-28"
    },
    {
        "id": 100021,
        "user": null,
        "restaurant": {
            "id": 100005,
            "name": "Burger King"
        },
        "date": "2020-08-28"
    },
    {
        "id": 100022,
        "user": null,
        "restaurant": 100005,
        "date": "2020-08-28"
    }
]

So first Vote for KFC shows full restaurant info, but second shows only ID. Same for Burger King which is next 2 votes.

What could be a problem?

You need to use com.fasterxml.jackson.annotation.JsonIdentityInfo annotation and declare it for Restaurant class:

@JsonIdentityInfo(generator = ObjectIdGenerators.None.class)
class Restaurant {

	private int id;
    ...
}

See also:

• Jackson/Hibernate, meta get methods and serialisation
• Jackson JSON - Using @JsonIdentityReference to always serialise a POJO by id