Spring如何存储有关2个实体之间关系的信息

Question

I have two entities that have these fields:

Code entity:

@Entity
@Table(name = "snippets")
@NoArgsConstructor
@Getter
@Setter
public class Code {

    private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");

    @Id
    @GeneratedValue
    private Long id;
    private String code;
    private String date;
    private User author;
    private List<User> allowedUsers;

    public Code(CodeDto code) {
        this.code = code.getCode();
        this.date = LocalDateTime.now().format(formatter);
        this.author = code.getAuthor();
    }
}

User entity:

@Entity
@Table(name = "users")
@Getter
@Setter
@EqualsAndHashCode
@NoArgsConstructor
public class User implements UserDetails {

    @Id
    @GeneratedValue
    private Long id;
    private String username;
    private String password;
    @Enumerated(EnumType.STRING)
    private UserRole userRole;
    @Transient
    private boolean locked;
    private boolean enabled = true;
    private List<Code> codeList;


    public User(String username, String password, UserRole userRole) {
        this.username = username;
        this.password = password;
        this.userRole = userRole;
    }
}

I am trying to create an application similar to github (but very simplified of course). I want each user to be able to add a code snippet, and later grant access to it to other users. I want to store the information who is the author and who has access to each code snippet. How can I achieve that?

Answer 1

• Each code can be seen by a list of users: That can be represented with a OneToMany relationship.
• Each user has a list of codes: That also is represented with a OneToMany relationship.

These relationships can be represented using this:

@OneToMany @JoinColumn(name = "code_id", referencedColumnName = "id")
private List<Code> codeList;

The same can be done for the other:

@OneToMany @JoinColumn(name = "user_id", referencedColumnName = "id")
private List<User> allowedUsers;

Answer 2

I want each user to be able to add a code snippet

I'm assuming you actually mean multiple code snippets, which would be more sensible. To achieve that you would need a OneToMany association between the User and Code, for which you would use @OneToMany.

You haven't mentioned the persistence provider you're using, but at least for Hibernate, I would suggest a bi-directional relation to avoid an extra update query when adding a Code.

When the relation is bi-directional, the @ManyToOne side of the relation, aka the owner side, is in charge of filling the foreign key column on the entity's insert, more here .

public class User implements UserDetails {
    ...
    @OneToMany(mappedBy = "writer")
    private Set<Code> codeList = new HashSet<>();

    public void addCode(Code code) {
        codeList.add(code);
        code.setWriter(this);
    }

    public void removeCode(Code code) {
        codeList.remove(code);
        code.setWriter(null);
    }

}

As we said, the association is bi-directional, so on the Code side, you would have:

public class Code {
    ...
    @ManyToOne(fetch = FetchType.LAZY)
    private User writer;
}

PS1: I switched list to set as it's the recommendation, morehere .

PS2: Dealing with bi-directional OneToMany relations can be tricky, ie forgetting to set the writer side in our example will result in a null foreign key in the database. A nice way to avoid that is to have helper methods to add and remove a child entity.

and later grant access to it to other users

For this, the association would be a @ManyToMany, in order to map it to a join table for in the database :

public class Code {
    ...
    @ManyToMany(cascade = {CascadeType.PERSIST,CascadeType.MERGE})
    @JoinTable(name = "code_viewers",
        joinColumns = @JoinColumn(name = "code_id"),
        inverseJoinColumns = @JoinColumn(name = "viewer_id"))
    private Set<User> viewers = new HashSet<>();
}

Once more, you need to set the other side of the relation:

public class User implements UserDetails {
    ...
    @ManyToMany(mappedBy = "viewers")
    private Set<Code> accessibleCodes = new HashSet<>();
}