How to make Spring Data REST fail on a DB constraint error on POST?

Question

I have a table with a composite primary key of the following structure:

create table follows (
    source_id           bigint              references users on delete cascade,
    target_id           bigint              references users on delete cascade,
    created_at          timestamptz         not null default now_utc(),

    primary key ( source_id, target_id ),
    constraint source_and_target_differ
        check ( source_id <> target_id )
);

However, when I try to create a new entity twice with 2 identical POST requests (which causes a constraint error on the DB side and it doesn't let the request through when I try to manually execute the SQL) it doesn't fail: it sends me the correct response first time, and an incorrect response all following times, but not an error.

My JPA entity looks the following way:

@Entity
@Table(name = "follows")
@IdClass(FollowPK.class)
@Follow.SourceAndTargetAreNotEqual // Custom validator
public class Follow {
    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "source_id", nullable = false)
    private User source;

    @Id
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "target_id", nullable = false)
    private User target;

    @Column(nullable = false, insertable = false, updatable = false)
    @Generated
    @Temporal(TemporalType.TIMESTAMP)
    private LocalDateTime createdAt;

    // Getters, Setters, etc by Lombok
}

public static final class FollowPK implements Serializable {
    private Long source;
    private Long target;
}

I was wondering if I could use some Hibernate/Spring setting to fix this, or maybe use @RepositoryEventHandler and @HandleBeforeSave