Spring JPA - There is way for keep multiple id with ID CLASS instead of Embedded?

Question

I have two Entities one for Forms and another one for Fields. I created a class for compose the primary key of Fields so I have 2 properties as Primary Key the Number and the foreign key Form. I want to insert just the foreign key with one post so I made the post request like that:

@CrossOrigin
    @PostMapping(value = "/field/{form_id}")
    public Field addField(@PathVariable Integer form_id, @RequestBody Field body) {
        Form result = formRepository.findById(form_id).orElse(null);
        body.setForm(result);
        fieldRepository.save(body);
        return body;
    }

My Form Class:

@Entity
public class Form {
        @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer id;
        @OneToMany(mappedBy="form", cascade = CascadeType.ALL)
    @OrderBy("id ASC")
        private Set<Field> fields;
.
.
.
(Setter & Getter)
}

My Field Class

@Entity
@IdClass(FieldKey.class)
@JsonIgnoreProperties(
        value = {"form"},
        allowSetters = true
)
public class Field {
        @Id
    private Integer id;
        @Id
        @ManyToOne
        @JoinColumn(name="form_id", nullable=false)
        private Form form;

.
.
.
(Setter and Getter)

My FieldKey class

@JsonIgnoreProperties(
        value = {"form"},
        allowSetters = true
)
public class FieldKey implements Serializable {

    private Integer id;
        private Form form;
    public FieldKey(Integer id, Form form) {
        this.id = id;
        this.form = form;
    }

    public FieldKey() {}

    (Setter & Getter)
}

I expected to insert into the field table the new field with the relation Form_ID but I get this error:

org.springframework.beans.ConversionNotSupportedException: Failed to convert property value of type 'java.lang.Integer' to required type 'com.model.Form' for property 'form'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.Integer' to required type 'com.model.Form' for property 'form': no matching editors or conversion strategy found

If I use the embedded Id annotation instead using the class as EmbeddedKey it works without any problem but I don't like the json annotation because it will be:

{"fieldKey":{"id":1,"form_id":28}, etc...}