M:M relationship results in empty set when fetching either object

Question

I have a many-to-many relationship between a Category and an Attribute object since every category can have many attributes.

This is my Category object:

@Data
@Entity
@Table(name = "categories")
public class Category {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "category_id")
    private Long id;

    @Column(nullable = false)
    private String name;

    @Column(name = "image_name", columnDefinition = "VARCHAR(255)")
    private String imageName;

    @Column(nullable = false)
    private Integer level;

    @OneToMany(mappedBy = "category")
    private List<Listing> listings;

    @OneToMany(mappedBy = "parentCategory", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Category> subcategories;
    @ManyToOne
    @JoinColumn(name = "parent_category_id")
    private Category parentCategory;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "categories_attributes",
            joinColumns = @JoinColumn(name = "category_id"),
            inverseJoinColumns = @JoinColumn(name = "attribute_id")
    )
    private Set<Attribute> attributes;
}

and this is the Attribute object:

@Data
@Entity
@Table(name = "attributes")
public class Attribute {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "attribute_id")
    private Long id;

    @Column
    private String name;

    @OneToMany(mappedBy = "attribute")
    private List<ListingDetail> listingDetails;

    @OneToMany(mappedBy = "attribute", cascade = CascadeType.ALL)
    private List<AttributeValue> attributeValues;

    @ManyToMany(mappedBy = "attributes")
    private Set<Category> categories;
}

When I try to fetch a category object from the database, the attributes field is always empty, even though it has valid records. The categories_attributes table looks like this:

category_id | attribute_id
    19             1
    19             2
    19             4
    19             8

I have a service layer, in which I map the Category object to a DAO response and before the mapping, the set is empty as well, so the problem isn't from the mapping process. This is the Category JSON response:


"id": 19,
"name": "Cars and Jeeps",
"imageName": null,
"level": 2,
"subcategories": [
     "id": 119,
     "name": "Acura",
     "imageName": null,
     "level": 3,
     "subcategories": [],
     "attributes": []
   },
   {
     "id": 120,
     "name": "Alfa romeo",
     "imageName": null,
     "level": 3,
     "subcategories": [],
     "attributes": []
   },
],
"attributes": []

What am I missing?

cheesus · Accepted Answer · 2024-06-12 22:06:47Z

0

So @Data indicates you are using lombok?

Are you sure this works without @Access(AccessType.FIELD) ?

Did you try calling getAttributes() before JSON serialization (which probably uses reflection and doesnt call the getter) ?

Cheers Tom.

answered Jun 12, 2024 at 22:06

cheesus

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1 Comment

zaro Over a year ago

I am not aware of how @Access(AccessType.FIELD) works and will check it out, but it turned out the problem was related to not having properly set @equals and @hashCode methods. The default ones from @Data didn't work and when I manually overrid them in Attribute class to use the id field, the problem got fixed. I am not sure what was the problem, but I guess it was related to HashSet since its the default implementation I think. Apparently the default hashCode and equals methods from @Data are not well set.

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