Spring Data JPA Self Join with Nullable foreign key

Question

I have problem with Spring Data JPA. I want to create a category that have parent category with the same table. Some categories don't need to have parent category, so parent category can be null. but when I do that, and save it, it shown error as below:

org.hibernate.TransientPropertyValueException: object references an unsaved 
transient instance - save the transient instance before flushing : 
com.ltech.solutions.ecom.models.category.Category.parent -> 
com.ltech.solutions.ecom.models.category.Category

Here is my model:

@Entity
@Table(name = "tb_category")
public class Category extends BaseEntity {

    @Column(name = "name", nullable = false)
    private String name;

    @JsonIgnore
    @ManyToOne(cascade = CascadeType.MERGE, fetch = FetchType.LAZY)
    @JoinColumn(name = "parent_id", referencedColumnName = "id")
    private Category parent;

    @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY)
    @JsonProperty(value = "sub_categories")
    private Set<Category> subCategories;

    @Column(name = "is_visible", insertable = false, columnDefinition = 
    "BOOLEAN DEFAULT TRUE")
    @JsonProperty(value = "is_visible")
    private Boolean isVisible;

    @Column(name = "is_enable", insertable = false, columnDefinition = "BOOLEAN 
    DEFAULT TRUE")
    @JsonProperty(value = "is_enable")
    private Boolean isEnable;

    @Column(name = "desc_attribute")
    private String descAttribute;
 }

• When saving object, I use save method of CRUDRepsoitory.

Here is my view:

<form th:object="${createCategoryForm}" th:action="@{/category/save}" 
method="post">
        <fieldset>
            <legend>Create Category</legend>
            <div class="form-group">
                <label>Name</label>
                <input class="form-control" th:field="*{name}" />
            </div>
            <div class="form-group">
                <label>Code</label>
                <input class="form-control" th:field="*{code}" />
            </div>
            <div class="form-group">
                <label>Description (English)</label>
                <input class="form-control" th:field="*{descEn}" />
            </div>
            <div class="form-group">
                <label>Description (Khmer)</label>
                <input class="form-control" th:field="*{descKh}" />
            </div>
            <div class="form-group">
                <label>Attribute</label>cs
                <input class="form-control" th:field="*{descAttribute}" />
            </div>
            <div class="form-group">
                <label>Parent of</label>
                <select class="form-control" th:field="*{parent.id}">
                    <option value="">None</option>
                    <option th:each="parent : ${parentCategories}" th:value="${parent.id}" th:text="${parent.name}"></option>
                </select>
            </div>
            <input type="submit" />
        </fieldset>
    </form>