How to attach a list of other entities to an entity and then work with them and with the list?

Question

Java

Hey!

There is a certificate (Certificate), it can have many tags (Tag).

Actually, I would like to understand how I can now work with its tags through the class of a specific Certificate? Get all his tags, change one/several? Add/remove new, etc.
Methods for this kind of started, in fact, before jpa there were lists so that RestController at least issued something and it was possible to write / read something at run-time.
How to stick the tag repository there - I'll never know.

repository here

base fields

....
@MappedSuperclass
public abstract class CommonFields implements Serializable {
  
  private static final long serialVersionUID = 5618898539406697796L;

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @JsonProperty("id")
  @Column(name = "id", nullable = false)
  private long id;

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

  @JsonProperty("dateCreated")
  @Column(name = "dateCreated", nullable = false)
  private ZonedDateTime dateCreated;

  @JsonProperty("dateUpdated")
  @Column(name = "dateUpdated", nullable = false)
  private ZonedDateTime dateUpdated;

}
...

Certificate

package by.alex.certws.domain;

@JsonSerialize
@Entity
@Table(name = "certificates")
public class Certificate extends CommonFields {
  
  private static final long serialVersionUID = -3444580092047125843L;
  
  @JsonProperty("content")
  private String content;

  @JsonProperty("tags")
  @OneToMany(targetEntity=Tag.class, mappedBy="id",cascade=CascadeType.ALL, fetch = FetchType.LAZY)    
  private List<Tag> tags = new ArrayList<>();

  public Certificate() {
    super();
  }

  public Certificate(long certId, String certName) {
    super(certId, certName);
  }

  public Certificate(long certId, String certName, String content) {
    super(certId, certName);
    this.setContent(content);
  }

  public Certificate(long id, String name, String content, String[] tags) {
    super(id, name);
    this.setContent(content);
    for (String tag : tags) {
      addTag(tag);
    }
  }

  public String getContent() {
    return this.content;
  }

  public void setContent(String target) {
    this.content = target;
    this.UpdateNotify();
  }

  public List<Tag> getTags() {
    return this.tags;
  }

  public Optional<Tag> findTagByName(String name) {
    return this.tags.stream().filter(Objects::nonNull)
        .filter(t -> (Objects.nonNull(t.getName()) && t.getName().compareTo(name) == 0)).findAny();
  }

  public long addTag(String tagName) {
    Optional<Tag> match = this.findTagByName(tagName);
    if (match.isPresent()) {
      return match.get().getId();
      // throw new ETagExistsException();
    } else {
      return 0;
    }
  }

  public Boolean removeTag(String target) {
    Optional<Tag> match = this.findTagByName(target);
    if (match.isPresent()) {
      return this.tags.remove(match.get());
    } else {
      // TODO: MUST throw exception
      return false;
    }
  }

}

Tag

package by.alex.certws.domain;

....

@Entity
@Table(name = "tags")
public class Tag extends CommonFields {
  
  @JsonProperty("references")
  @OneToMany(targetEntity=Certificate.class, mappedBy="id",cascade=CascadeType.ALL, fetch = FetchType.LAZY)    
  private List<Certificate> refs = new ArrayList<>();
  
  private static final long serialVersionUID = -2535451222904563266L;

  public Tag() {
    super();
  }

  public Tag(long id, String name) {
    super(id, name);
  }
  
  List<Certificate> getReferences() {
    return this.refs;
  }

}

There are also interfaces

public interface CertificatesRepository extends PagingAndSortingRepository<Certificate, Long> {}
...
public interface TagsRepository extends PagingAndSortingRepository<Tag, Long> {}

It is assumed that in the tags table there may be a bunch of duplicates by name, i.e. 3 certificates can have the same "Java" tag or something like that.
I still don’t quite understand how and why, but usually I create tagsIDS for such a table with my hands where the Autokey connection will actually live, Certificate.id = Tag.id and now it’s not there.
but here everything is through .... automation / simplification and conversion of classes and properties to sql, etc.

Answer 1

Good afternoon!
First, based on your words:

There is a certificate (Certificate), it can have many tags (Tag).
It is assumed that in the tags table there may be a bunch of duplicates by name, i.e. 3 certificates can have the same "Java" tag or something like that.

I can assume that the OneToMany & ManyToOne relationship is not very suitable.
Here, look: one certificate can have several tags, but one tag can refer to different certificates. There is clearly a ManyToMany connection. https
://vladmihalcea.com/the-best-way-to-use-the-m...
https://www.baeldung.com/jpa-many-to-many
you can get a list of its tags, and from the tag side, you can get a list of certificates to which it belongs.
Otherwise, it turns out that there is a connection from the side of the tag and the OneToMany certificate, but at the same time List is used on both sides.
When using ManyToMany, a new table of the form will be created certificate_id | tag_idwhere you can store data. And correspondingly,

How to attach a list of other entities to an entity and then work with them and with the list?

This issue will resolve itself.