JPA - Get collection from ManyToOne relationship

Question

I have a table Post and Post_Image

 @Entity
@Table(name = "post")
@XmlRootElement
@NamedQueries({
    @NamedQuery(name = "Post.findAll", query = "SELECT p FROM Post p"),
    @NamedQuery(name = "Post.findByPostId", query = "SELECT p FROM Post p WHERE p.postId = :postId"),
    @NamedQuery(name = "Post.findByTitle", query = "SELECT p FROM Post p WHERE p.title = :title"),
    @NamedQuery(name = "Post.findByCreatedDatetime", query = "SELECT p FROM Post p WHERE p.createdDatetime = :createdDatetime")})
public class Post implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @NotNull
    @Column(name = "post_id")
    private Integer postId;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 500)
    @Column(name = "title")
    private String title;
    @Basic(optional = false)
    @NotNull
    @Lob
    @Size(min = 1, max = 65535)
    @Column(name = "content")
    private String content;
    @Column(name = "created_datetime")
    @Temporal(TemporalType.TIMESTAMP)
    private Date createdDatetime;
    @JoinColumn(name = "user_id", referencedColumnName = "user_id")
    @ManyToOne(optional = false)
    private User userId;
    @JoinColumn(name = "post_type_id", referencedColumnName = "post_type_id")
    @ManyToOne(optional = false)
    private PostType postTypeId;

    public Post() {
        Date date = new Date();
        this.createdDatetime =new Date(date.getTime());

    }

    public Post(Integer postId) {
        this.postId = postId;
    }

    public Post(Integer postId, String title, String content) {
        this.postId = postId;
        this.title = title;
        this.content = content;
    }

    public Integer getPostId() {
        return postId;
    }

    public void setPostId(Integer postId) {
        this.postId = postId;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

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

    public Date getCreatedDatetime() {
        return createdDatetime;
    }

    public void setCreatedDatetime(Date createdDatetime) {
        this.createdDatetime = createdDatetime;
    }

    public User getUserId() {
        return userId;
    }

    public void setUserId(User userId) {
        this.userId = userId;
    }

    public PostType getPostTypeId() {
        return postTypeId;
    }

    public void setPostTypeId(PostType postTypeId) {
        this.postTypeId = postTypeId;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (postId != null ? postId.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Post)) {
            return false;
        }
        Post other = (Post) object;
        if ((this.postId == null && other.postId != null) || (this.postId != null && !this.postId.equals(other.postId))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "entity.Post[ postId=" + postId + " ]";
    }

}

and

@Entity
@Table(name = "post_image")
@XmlRootElement
@NamedQueries({
    @NamedQuery(name = "PostImage.findAll", query = "SELECT p FROM PostImage p"),
    @NamedQuery(name = "PostImage.findByPostImageId", query = "SELECT p FROM PostImage p WHERE p.postImageId = :postImageId"),
    @NamedQuery(name = "PostImage.findByPath", query = "SELECT p FROM PostImage p WHERE p.path = :path"),
    @NamedQuery(name = "PostImage.findByTitle", query = "SELECT p FROM PostImage p WHERE p.title = :title")})
public class PostImage implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "post_image_id")
    private Integer postImageId;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 500)
    @Column(name = "path")
    private String path;
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 500)
    @Column(name = "title")
    private String title;
    @JoinColumn(name = "post_id", referencedColumnName = "post_id")
    @ManyToOne(optional = false)
    private Post postId;

    public PostImage() {
    }

    public PostImage(Integer postImageId) {
        this.postImageId = postImageId;
    }

    public PostImage(Integer postImageId, String path, String title) {
        this.postImageId = postImageId;
        this.path = path;
        this.title = title;
    }

    public Integer getPostImageId() {
        return postImageId;
    }

    public void setPostImageId(Integer postImageId) {
        this.postImageId = postImageId;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public Post getPostId() {
        return postId;
    }

    public void setPostId(Post postId) {
        this.postId = postId;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (postImageId != null ? postImageId.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof PostImage)) {
            return false;
        }
        PostImage other = (PostImage) object;
        if ((this.postImageId == null && other.postImageId != null) || (this.postImageId != null && !this.postImageId.equals(other.postImageId))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "entity.PostImage[ postImageId=" + postImageId + " ]";
    }

}

i want to get collection of images for particular post like Collection objPostImage = objPost.getPostImageCollection() but manytoone relationship do not provide this functionality to me how can i convert it to one to many or how can i get Image Collection for a post.? I am new to java so any help and suggestion will be appreciated thanx in advance...

score 0 · Answer 1 · edited Jun 20 '20 at 09:12

You can add a java.util.Set of PostImages in your Post object, and use the Hibernate mapping to provide the relationship. This site has a great example of setting up One to Many relationships.

So, for example, you would want to add something like the following to your Post class:

private Set<PostImage> postImages = new HashSet<PostImage>();

@OneToMany(fetch = FetchType.LAZY, mappedBy = "post")
public Set<PostImage> getPostImages() {
return this.postImages;
}

public void setPostImages(Set<PostImage> postImages) {
this.postImages= postImages;
}

Then, in your PostImage class, add a reference to a Post object:

private Post post;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "POST_ID", nullable = false)
public Stock getPost() {
return this.post;
}

public void setPost(Post post) {
this.post= post;
}

After adding that, you will be able to call the getPostImages() method on your Post object.

Not working i got this error Internal Exception: Exception [EclipseLink-7244] (Eclipse Persistence Services - 2.3.2.v20111125-r10461): org.eclipse.persistence.exceptions.ValidationException Exception Description: An incompatible mapping has been encountered between [class entity.Post] and [class entity.PostImage]. This usually occurs when the cardinality of a mapping does not correspond with the cardinality of its backpointer.. — Jitendra, Jun 05 '12 at 18:02

score 0 · Answer 2 · answered Jun 07 '12 at 06:42

Try this:

@Entity
@Table(name = "post")
public class Post
{
    //....

    @OneToMany(mappedBy = "post")
    private Set<PostImage> images;

    //....
}

@Entity
@Table(name = "post_image")
public class PostImage
{
    //....

    @JoinColumn(name = "post_id", referencedColumnName = "id")
    @ManyToOne(optional = false)
    private Post post;

    //....
}

The reason why Seth's answer didn't work is because EclipseLink uses fields to access persistence data. (Hibernate uses properties IIRC.) You can specify per class how a JPA provider should access this data.

Using fields:

@Entity
@Access(AccessType.FIELD)
public class SomeEntity
{
    @Id
    private Long id;

    //....
}

Using properties:

@Entity
@Access(AccessType.PROPERTY)
public class SomeEntity
{
    private Long id;

    //....

    @Id
    public Long getId()
    {
        return id;
    }
}

However when using @Access(AccessType.PROPERTY) fields are also used (at least in EclipseLink) so something like this is possible:

@Entity
@Access(AccessType.PROPERTY)
public class SomeEntity
{
    private Long id;

    @Column(name = "text")
    private String someText;

    //....

    @Id
    public Long getId()
    {
        return id;
    }
}

JPA - Get collection from ManyToOne relationship

2 Answers2