使用 JPA 和 MySQL 检查用户是否喜欢帖子的最佳方法是什么？

【问题标题】：What would be the best way to implement to check if the Post has been liked by the User using JPA and MySQL?使用 JPA 和 MySQL 检查用户是否喜欢帖子的最佳方法是什么？
【发布时间】：2016-10-26 03:10:48
【问题描述】：

好吧，这听起来可能很愚蠢，但我是 MySQL 的新手，所以我真的不知道这是否是 Joins 或其他东西的情况。

所以我有 3 个表，POST、USER、LIKES。基本上他们有：

POST
post_id   |   caption   |   media   |   ...

USER
user_id   |   name      |   email   |   ...

LIKES
like_id   |   post_id   |   user_id |   ...

Java 中的对象是：

@Entity
@Table(name="POST")
public class PostMW{

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

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name="media_id", nullable=false, unique=true)
    private MediaMW media;

    ....

    @Transient
    private Boolean liked;

}

喜欢：

@Entity
@Table(name="LIKES")
public class LikeMW {

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

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "post_id", nullable=false)
    @JsonBackReference
    private PostMW post;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name="user_id", nullable=false)
    private UserMW user;

}

用户可以请求 POST 列表，因此如果该用户喜欢它，我需要在每个 POST 的“喜欢”属性中设置。

到目前为止，我做到了：

public class FeedServiceImpl implements FeedService{
    private PostDao<PostMW, Long> postDao;

    ...

    @Override
    public List<PostMW> getFeed(int page, UserMW user) {
        List<PostMW> result;
        int offset = page * ConstUtil.FEED_POSTS_PER_PAGE;
        postDao.openCurrentSession();
        result = postDao.list(offset, ConstUtil.FEED_POSTS_PER_PAGE);

        for(PostMW post : result){
            //TODO
            //post.setLiked(postDao.postLikedByUser(post, user));

            //PS
            // FOR EVERY POST I'M EXECUTING THE QUERY BELOW

            //@NamedQuery(name="isLiked", query="SELECT COUNT(likes.id) FROM 
            //LikeMW likes WHERE likes.post.id = :postID AND likes.user.id = :userID"),
        }

        postDao.closeCurrentSession();

        return result;
    }

    ...

}

我什至不知道它是否有效，因为我真的不喜欢检索例如 100 个帖子并在循环内连续发出 100 个请求以检查用户是否喜欢该帖子的想法我什至没有尝试。

那么，我该怎么办？任何建议将不胜感激！

谢谢！

编辑：

现在尝试这个查询：

List<Object> result = entityManager
                .createQuery("SELECT post FROM PostMW post LEFT OUTER JOIN post.hasLiked liked ON post.id = liked.post_id AND liked.user_id =:userID")
                .setParameter("userID", 1)
                .setFirstResult(offset).setMaxResults(itemsPerPage)
                .getResultList();

更新了我的对象。现在我的 PostMW 对象有 

@Entity
@Table(name="POST")
public class PostMW{
    ...
    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name="LIKES", joinColumns=@JoinColumn(name="post_id"), inverseJoinColumns=@JoinColumn(name="user_id"))
    private List<UserMW> hasLiked;
    ...
}

而 UserMW 有

@Entity
@Table(name="USER")
public class UserMW {
    ...
    @Id
    @Column(name="id", nullable=false)
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    @Column(name="name", nullable=false, length=255)
    private String name;
    ...
}

得到这个异常：

java.lang.IllegalArgumentException: An exception occurred while creating a query in EntityManager: 
Exception Description: Internal problem encountered while compiling [SELECT post FROM PostMW post LEFT OUTER JOIN LikeMW likes ON post.id = likes.post_id AND likes.user_id =:userID].
Internal Exception: java.lang.NullPointerException
at org.eclipse.persistence.internal.jpa.EntityManagerImpl.createQuery(EntityManagerImpl.java:1605)
at com.jonathan.myworld.model.dao.impl.PostDaoImpl.list(PostDaoImpl.java:72)
at com.jonathan.myworld.service.impl.FeedServiceImpl.getFeed(FeedServiceImpl.java:45)
at com.jonathan.myworld.UserWS.login(UserWS.java:151)
at com.jonathan.myworld.UserWS.loginWithGoogle(UserWS.java:60)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.google.appengine.tools.development.agent.runtime.Runtime.invoke(Runtime.java:130)
at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:363)
at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:113)
at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:71)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:128)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:50)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectRequest(DevAppServerModulesFilter.java:366)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectModuleRequest(DevAppServerModulesFilter.java:349)
at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:116)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:511)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.1.v20130918-f2b9fc5): org.eclipse.persistence.exceptions.JPQLException
Exception Description: Internal problem encountered while compiling [SELECT post FROM PostMW post LEFT OUTER JOIN LikeMW likes ON post.id = likes.post_id AND likes.user_id =:userID].
Internal Exception: java.lang.NullPointerException
at org.eclipse.persistence.internal.jpa.jpql.HermesParser.buildUnexpectedException(HermesParser.java:207)
at org.eclipse.persistence.internal.jpa.jpql.HermesParser.populateQueryImp(HermesParser.java:296)
at org.eclipse.persistence.internal.jpa.jpql.HermesParser.buildQuery(HermesParser.java:163)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:142)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.buildEJBQLDatabaseQuery(EJBQueryImpl.java:116)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:102)
at org.eclipse.persistence.internal.jpa.EJBQueryImpl.<init>(EJBQueryImpl.java:86)
at org.eclipse.persistence.internal.jpa.EntityManagerImpl.createQuery(EntityManagerImpl.java:1603)
... 49 more
Caused by: java.lang.NullPointerException
at org.eclipse.persistence.queries.DatabaseQuery.addArgument(DatabaseQuery.java:449)
at org.eclipse.persistence.queries.DatabaseQuery.addArgument(DatabaseQuery.java:419)
at org.eclipse.persistence.internal.jpa.jpql.HermesParser.addArguments(HermesParser.java:98)
at org.eclipse.persistence.internal.jpa.jpql.HermesParser.populateQueryImp(HermesParser.java:287)

【问题讨论】：

    标签： java mysql jpa join orm


    【解决方案1】：

    您可以尝试从您刚刚收到的帖子列表中提取喜欢的帖子。带有“SELECT ... IN”请求），然后填充布尔值 isLiked；类似：

     ...

@Override
public List<PostMW> getFeed(int page, UserMW user) {
    List<PostMW> result;
    int offset = page * ConstUtil.FEED_POSTS_PER_PAGE;
    postDao.openCurrentSession();
    result = postDao.list(offset, ConstUtil.FEED_POSTS_PER_PAGE);

    List<Long> postIds = new ArrayList();

    for(PostMW post : result){           
       postIds.add(post.getid();
    }

   //then EXtract all liked post from list :
   Query queryIn = em.createQuery("SELECT p.post.id FROM LikeMW p WHERE p.user = :user AND p.post IN :values");

   queryIn.setParameter("values", postIds );
   queryIn.setParameter("user", currentUser);
   List<Object[]> resultIn = queryIn.getResultList();

   Set<Long> postIdLiked ) new HashSet<>();
   for(Object[] row: resultIn) {
       postIdLiked.add(row[0]);
   }
   //then fill the boolean isLiked  
   for(PostMW post : result){            
       if (postIdLiked.contains(post.getId()) 
          post.setLiked(true);      
   }

    postDao.closeCurrentSession();
    return result;
}

...

    【讨论】：

    • 这可能是最好的答案，但我仍然需要进行 2 次数据库调用（我忘了提到我正在从 GAE 调用 Amazon RDS，所以最少的调用我会做得最好)，经过 3 个循环，每个循环可能有 100 个项目，并且在数据库中有一种“循环”......希望有更好的方法。无论如何，谢谢你的回答。如果没有其他人回复，我会将其标记为正确的。
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