translate SQL join to JPQL - jpa

Question

Currently, all articles in the join for the JPQL query are returned in the getRangeOfArticles method, whereas only a range of articles should get returned.
The MySQL console output I'm after, but would like to implement in JPQL:
mysql>
mysql> SELECT * FROM articles LEFT OUTER JOIN newsgroups ON articles.NEWSGROUP_ID=newsgroups.ID WHERE newsgroups.NEWSGROUP = "gwene.com.economist" AND articles.ID BETWEEN 450 AND 453;
+-----+---------------+--------------+------+---------------------+
| ID | MESSAGENUMBER | NEWSGROUP_ID | ID | NEWSGROUP |
+-----+---------------+--------------+------+---------------------+
| 450 | 554 | 5 | 5 | gwene.com.economist |
| 451 | 555 | 5 | 5 | gwene.com.economist |
| 452 | 556 | 5 | 5 | gwene.com.economist |
| 453 | 557 | 5 | 5 | gwene.com.economist |
+-----+---------------+--------------+------+---------------------+
4 rows in set (0.02 sec)
mysql>
mysql> show tables;
+------------------+
| Tables_in_usenet |
+------------------+
| articles |
| newsgroups |
+------------------+
2 rows in set (0.01 sec)
mysql> describe articles;
+---------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+------------+------+-----+---------+----------------+
| ID | bigint(20) | NO | PRI | NULL | auto_increment |
| MESSAGENUMBER | int(11) | YES | | NULL | |
| NEWSGROUP_ID | bigint(20) | YES | MUL | NULL | |
+---------------+------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> describe newsgroups;
+-----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+----------------+
| ID | bigint(20) | NO | PRI | NULL | auto_increment |
| NEWSGROUP | varchar(255) | YES | | NULL | |
+-----------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql>
I'm trying to replicate that SQL query in JPQL in the getRangeOfArticles method:
package net.bounceme.dur.usenet.driver;
import java.util.List;
import java.util.logging.Logger;
import javax.mail.Folder;
import javax.mail.Message;
import javax.persistence.*;
import net.bounceme.dur.usenet.model.Article;
import net.bounceme.dur.usenet.model.Newsgroup;
class DatabaseUtils {
private static final Logger LOG = Logger.getLogger(DatabaseUtils.class.getName());
private EntityManagerFactory emf = Persistence.createEntityManagerFactory("USENETPU");
private EntityManager em = emf.createEntityManager();
//SELECT MAX(MESSAGENUMBER) FROM articles LEFT OUTER JOIN newsgroups ON articles.NEWSGROUP_ID=newsgroups.ID WHERE newsgroups.NEWSGROUP = "gwene.com.economist";
public int getMaxMessageNumber(Folder folder) {
int maxMessageNumber = 0;
String newsgroup = folder.getFullName();
String queryString = "select max(article.messageNumber) from Article article left join article.newsgroup newsgroup where newsgroup.newsgroup = '" + newsgroup + "'";
try {
maxMessageNumber = (Integer) em.createQuery(queryString).getSingleResult();
} catch (Exception e) {
LOG.severe("setting max to zero for " + newsgroup);
}
LOG.fine(folder.getFullName() + "\t" + maxMessageNumber);
return maxMessageNumber;
}
//SELECT * FROM articles LEFT OUTER JOIN newsgroups ON articles.NEWSGROUP_ID=newsgroups.ID WHERE newsgroups.NEWSGROUP = "gwene.com.economist" AND articles.ID BETWEEN 450 AND 500;
public List<Article> getRangeOfArticles(Page page) {
String fullNewsgroupName = page.getFolder().getFullName();
int minRange = page.getMin();
int maxRange = page.getMax();
String queryString = "select article from Article article left join article.newsgroup newsgroup where newsgroup.newsgroup = :newsGroupParam";
TypedQuery<Article> query = em.createQuery(queryString, Article.class);
query.setParameter("newsGroupParam", fullNewsgroupName);
List<Article> results = query.getResultList();
LOG.info(results.toString());
return results;
}
public void persistArticle(Message message, Folder folder) {
em.getTransaction().begin();
String fullNewsgroupName = folder.getFullName();
Newsgroup newsgroup = null;
TypedQuery<Newsgroup> query = em.createQuery("SELECT n FROM Newsgroup n WHERE n.newsgroup = :newsGroupParam", Newsgroup.class);
query.setParameter("newsGroupParam", fullNewsgroupName);
try {
newsgroup = query.getSingleResult();
LOG.fine("found " + query.getSingleResult());
} catch (javax.persistence.NoResultException e) {
LOG.fine(e + "\ncould not find " + fullNewsgroupName);
newsgroup = new Newsgroup(folder);
em.persist(newsgroup);
} catch (NonUniqueResultException e) {
LOG.warning("\nshould never happen\t" + fullNewsgroupName);
} /*
* finally { if (em.getTransaction().isActive()) {
* em.getTransaction().rollback(); }
*/
Article article = new Article(message, newsgroup);
em.persist(article);
em.getTransaction().commit();
}
public void close() {
em.close();
emf.close();//necessary?
}
}

Answer 1

If you want to query a collection of Articles by their Newsgroup's name you can do something like this:
String queryString = "select a from Article a where a.newsgroup.newsgroup = :newsGroupParam";
TypedQuery<Article> query = em.createQuery(queryString, Article.class);
query.setParameter("newsGroupParam", fullNewsgroupName);
List<Article> results = query.getResultList();
Don't write joins in JPQL, let JPA do the work for you.
As an extra tip, you should always write your queries like this to prevent SQL injection.
public int getMaxMessageNumber(Folder folder) {
int maxMessageNumber = 0;
String queryString = "select max(a.messageNumber) from Article a where a.newsgroup.newsgroup = :newsgroup";
Query q = em.createQuery(queryString);
q.setParameter("newsgroup", folder.getFullName());
try {
maxMessageNumber = (Integer) q.getSingleResult();
} catch (Exception e) {
LOG.severe("setting max to zero for " + newsgroup);
}
LOG.fine(folder.getFullName() + "\t" + maxMessageNumber);
return maxMessageNumber;
}

Answer 2

Just in terms of the query, I believe it's String queryString = "select article from Article article left join article.newsgroup newsgroup where newsgroup.newsgroup = :newsGroupParam and article.messageNumber between 100 and 500"; although I need to add some parameters in there for ranges. See getRangeOfArticles below:
package net.bounceme.dur.usenet.driver;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.mail.Folder;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.persistence.*;
import net.bounceme.dur.usenet.model.Article;
import net.bounceme.dur.usenet.model.Newsgroup;
import net.bounceme.dur.usenet.model.Usenet;
class DatabaseUtils {
private static final Logger LOG = Logger.getLogger(DatabaseUtils.class.getName());
private EntityManagerFactory emf = Persistence.createEntityManagerFactory("USENETPU");
private EntityManager em = emf.createEntityManager();
//SELECT MAX(MESSAGENUMBER) FROM articles LEFT OUTER JOIN newsgroups ON articles.NEWSGROUP_ID=newsgroups.ID WHERE newsgroups.NEWSGROUP = "gwene.com.economist";
public int getMaxMessageNumber(Folder folder) {
int maxMessageNumber = 0;
String newsgroup = folder.getFullName();
String queryString = "select max(article.messageNumber) from Article article left join article.newsgroup newsgroup where newsgroup.newsgroup = '" + newsgroup + "'";
try {
maxMessageNumber = (Integer) em.createQuery(queryString).getSingleResult();
} catch (Exception e) {
LOG.severe("setting max to zero for " + newsgroup);
}
LOG.fine(folder.getFullName() + "\t" + maxMessageNumber);
return maxMessageNumber;
}
//SELECT * FROM articles LEFT OUTER JOIN newsgroups ON articles.NEWSGROUP_ID=newsgroups.ID WHERE newsgroups.NEWSGROUP = "gwene.com.economist" AND articles.ID BETWEEN 450 AND 500;
public List<Article> getRangeOfArticles(Page page) {
String fullNewsgroupName = page.getFolder().getFullName();
int minRange = page.getMin();
int maxRange = page.getMax();
String queryString = "select article from Article article left join article.newsgroup newsgroup where newsgroup.newsgroup = :newsGroupParam and article.messageNumber between 100 and 500";
TypedQuery<Article> query = em.createQuery(queryString, Article.class);
query.setParameter("newsGroupParam", fullNewsgroupName);
List<Article> articles = query.getResultList();
Usenet usenet = Usenet.INSTANCE;
for (Article article : articles) {
List<String> string = new ArrayList<>();
string.add(article.getId().toString());
string.add(Long.toString(article.getMessageNumber()));
Message message = usenet.getMessage(page.getFolder(), article.getMessageNumber());
try {
string.add(message.getSubject());
} catch (MessagingException ex) {
Logger.getLogger(DatabaseUtils.class.getName()).log(Level.SEVERE, null, ex);
}
Newsgroup n = article.getNewsgroup();
string.add(n.getNewsgroup());
LOG.info(string.toString());
}
return articles;
}
public void persistArticle(Message message, Folder folder) {
em.getTransaction().begin();
String fullNewsgroupName = folder.getFullName();
Newsgroup newsgroup = null;
TypedQuery<Newsgroup> query = em.createQuery("SELECT n FROM Newsgroup n WHERE n.newsgroup = :newsGroupParam", Newsgroup.class);
query.setParameter("newsGroupParam", fullNewsgroupName);
try {
newsgroup = query.getSingleResult();
LOG.fine("found " + query.getSingleResult());
} catch (javax.persistence.NoResultException e) {
LOG.fine(e + "\ncould not find " + fullNewsgroupName);
newsgroup = new Newsgroup(folder);
em.persist(newsgroup);
} catch (NonUniqueResultException e) {
LOG.warning("\nshould never happen\t" + fullNewsgroupName);
} /*
* finally { if (em.getTransaction().isActive()) {
* em.getTransaction().rollback(); }
*/
Article article = new Article(message, newsgroup);
em.persist(article);
em.getTransaction().commit();
}
public void close() {
em.close();
emf.close();//necessary?
}
}
However, I seem to get some errors:
Aug 06, 2012 12:00:02 AM net.bounceme.dur.usenet.model.Usenet getMessage
SEVERE: null
javax.mail.MessageRemovedException: No such article: 273
so perhaps there's an error with the query syntax or meaning, or I have a logical error.