Error when merging manyToMany relation in JPA

[David Freed]

I have a JPA project which searches tweets by keword and should persist the key - tweet pairs in the database. The project has the following entities :

    @Entity
    @Table(name="keywords")
    
    public class Keyword implements Serializable {
    	private static final long serialVersionUID = -1429681347817644570L;
    
    	@Id
    	@GeneratedValue(strategy=GenerationType.AUTO)
    	@Column(name="key_id")
    	private int keyId;   
    
    	@Column(name="key_name",unique=true)
    	private String key_name;   
    
    
    	@ManyToMany(cascade=CascadeType.PERSIST)
    	@JoinTable(
    		 name="tweet_keyword",
    		 joinColumns={@JoinColumn(name="key_id",
            referencedColumnName="key_id")},
    		 inverseJoinColumns={@JoinColumn(name="tweet_id",
          referencedColumnName="tweet_id")}
    		      )
    	private Set<Tweet> tweets;
	
    

**Tweet** entity

    @Entity
    @Table(name="tweets")
    
    public class Tweet implements Serializable{
    
    	private static final long serialVersionUID = -1041037108182045708L;
    	@Id
    	@Column(name="tweet_id")
    	private long tweetId;
    
    	@Column(name="tweet_text")
    	private String tweetText;
    
    	@Temporal(TemporalType.TIMESTAMP)
    	@Column(name = "created_at", nullable = false, length = 19)
    	private Date createdAt;
    
    	@Column(name="lang_code")
    	private String languageCode;
    
    	@ManyToOne(cascade=CascadeType.PERSIST,optional=false)
    	@JoinColumn(name = "user_id",referencedColumnName="user_id")
    	private User user;
    	
    
    	@ManyToMany(cascade=CascadeType.PERSIST)
    	@JoinTable(
    		 name="tweet_hashtag",
    		 joinColumns={@JoinColumn(name="tweet_id", referencedColumnName="tweet_id")},
    		 inverseJoinColumns={@JoinColumn(name="hashtag_id", referencedColumnName="hashtag_id")}
    		      )
    	private Set<Hashtag> hashtags;
    
    	
    	@ManyToMany(cascade=CascadeType.PERSIST)
    	@JoinTable(
    		 name="tweet_url",
    		 joinColumns={@JoinColumn(name="tweet_id", referencedColumnName="tweet_id")},
    		 inverseJoinColumns={@JoinColumn(name="url_id", referencedColumnName="url_id")}
    		      )
    	private Set<Url> urls;

Now I have a class for persisting a Keyword,

    public class TweetPersistent {
    
    	private EntityManagerFactory emf;
    	
    	private EntityManager em;
    	
    
    	public TweetPersistent()
    	{
    		emf=Persistence.createEntityManagerFactory("mypu");
    		em=emf.createEntityManager();
    	}
    
	    em.getTransaction().begin();

		Keyword oldKeyword=this.findKeyword(keyword.getKeyName());
		
	        
		if(oldKeyword!=null)
		{
			keyword.setKeyId(oldKeyword.getKeyId());
			em.merge(keyword);
		    em.getTransaction().commit();
		}
		else
		{
	        em.persist(keyword);
	        em.getTransaction().commit();
		}


    	
    	private Keyword findKeyword(String key)
    	{
    		return (Keyword) em.createQuery("SELECT k FROM keyword k where 
           k.key_name = :value1")
                    .setParameter("value1", key).getSingleResult();
    	}
    	
    	
    }

As you can see in the Keyword Entity, it has a joinTable and therefore a field that can have a set of tweets.

So whenever I get a key and a set of tweets, first I make a user entity, a set of hashtags and urls out of that tweet. Then I have a new tweet entity object.
In the next step I add this tweet(these tweets) to the keyword entity object.
Now I persist this keyword.

Problem: When I want to search for a keyword for the next time, the keyword exists and I should merge, but I get the following error :

    Picked up JAVA_TOOL_OPTIONS: -javaagent:/usr/share/java/jayatanaag.jar 
    log4j:WARN No appenders could be found for logger (org.jboss.logging).
    log4j:WARN Please initialize the log4j system properly.
    log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
    Exception in thread "Thread-3" javax.persistence.EntityNotFoundException: Unable to find model.twitter.entities.Tweet with id 619564340417576960
    	at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl$JpaEntityNotFoundDelegate.handleEntityNotFound(EntityManagerFactoryBuilderImpl.java:183)
    	at org.hibernate.event.internal.DefaultLoadEventListener.load(DefaultLoadEventListener.java:219)
    	at org.hibernate.event.internal.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:275)
    	at org.hibernate.event.internal.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:151)
    	at org.hibernate.internal.SessionImpl.fireLoad(SessionImpl.java:1070)
    	at org.hibernate.internal.SessionImpl.internalLoad(SessionImpl.java:989)
    	at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:716)
    	at org.hibernate.type.EntityType.resolve(EntityType.java:502)
    	at org.hibernate.type.EntityType.replace(EntityType.java:366)
    	at org.hibernate.type.CollectionType.replaceElements(CollectionType.java:549)
    	at org.hibernate.type.CollectionType.replace(CollectionType.java:690)
    	at org.hibernate.type.TypeHelper.replace(TypeHelper.java:177)
    	at org.hibernate.event.internal.DefaultMergeEventListener.copyValues(DefaultMergeEventListener.java:407)
    	at org.hibernate.event.internal.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:344)
    	at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:186)
    	at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:85)
    	at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:876)
    	at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:858)
    	at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:863)
    	at org.hibernate.jpa.spi.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:1196)
    	at model.twitter.TweetPersistent.insertResults(TweetPersistent.java:47)
    	at controller.TweetController.save(TweetController.java:77)
    	at controller.TweetController.parse(TweetController.java:48)
    	at main.TwitterStreamConsumer.run(TwitterStreamConsumer.java:26)
    	at java.lang.Thread.run(Thread.java:745)

Thank you so much in advance. I'm waiting here.