I have this entity that represents products stocked:

@Id
    private String lotNumber;

    @Id
    @ManyToOne(cascade = CascadeType.MERGE)
    @JoinColumn(name = "productId", foreignKey = @ForeignKey(name = "fk_product"), referencedColumnName = "productId")
    private Products product;

    @Id
    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumns({
            @JoinColumn(name = "id", foreignKey = @ForeignKey(name = "fk_billId")),
            @JoinColumn(name = "supplier", foreignKey = @ForeignKey(name = "fk_billSupplier"))
    })

    private Bills bills;

Now everything works fine when presisting entities but when i fetch them in JSON format this is what i get:

 "@lotNumber": "537fe03f-b261-48bb-b1dd-9215b5ad868a",
        "lotNumber": "LOT001",
        "product": {},
        "bills": 7,

The product serialized as expected i removed its content to keep the message short, what is weird is how the bills entity serialized as just its Id from JsonIdentity().
I can’t find any mention of this behavior so can somebody explain why does this exactly happen?