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POJPA with Hibernate - merge performance issues
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  1. POJPA with Hibernate - merge performance issues
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    copied!<p>I stumbled on a problem with <code>merge</code> method of <code>EntityManager</code>.</p> <p>I have this class</p> <pre><code>@Entity public class QuestionList { @Id @GeneratedValue(strategy = GenerationType.AUTO) private Long id; private String name; @ManyToOne private UserEntity user; @OneToMany(cascade = CascadeType.ALL) private List&lt;QuestionUnit&gt; units = new ArrayList&lt;QuestionUnit&gt;(); public List&lt;QuestionUnit&gt; getUnits() { return units; } public void setUnits(List&lt;QuestionUnit&gt; units) { this.units = units; } public Long getId() { return id; } public void setName(String name) { this.name = name; } public String getName() { return name; } public void addQuestionUnit(QuestionUnit QuestionUnit) { this.units.add(QuestionUnit); } public void setUser(UserEntity user) { this.user = user; } public UserEntity getUser() { return this.user; } } </code></pre> <p>now, if I just modify one of the objects in <code>units</code> the merge is fine, but if I delete or add new object to the collection, the way that merge happens is that I am getting <code>delete from QuestionList_QuestionUnit where QuestionList_id=?</code> and then every row is added one by one <code>insert into QuestionList_QuestionUnit (QuestionList_id, units_id) values (?, ?)</code></p> <p>I am wondering, is it possible to tell JPA/Hibernate to just add/delete the row associated with the object added/deleted? Or is it the way that merge just works with collections and in my case I need to write my own query for updating the database.</p> <p>I am asking because this collection can be potentially pretty huge, and just deleting all rows and inserting almost all of them again doesn't seems like a good way to update the table.</p> <p>EDIT: as requested:</p> <pre><code>@Entity @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) public abstract class QuestionUnit { @Id @GeneratedValue(strategy = GenerationType.TABLE) private long id; @OneToOne(cascade = CascadeType.ALL) private AnswerUnit correctAnswer; public QuestionUnit(AnswerUnit correctAnswer) { this.correctAnswer = correctAnswer; } public void setCorrectAnswer(AnswerUnit correctAnswer) { this.correctAnswer = correctAnswer; } public AnswerUnit getCorrectAnswer() { return this.correctAnswer; } public long getId() { return id; } public void setId(long id){ this.id=id; } public abstract Object getQuestionContent(); public abstract String getTitle(); } </code></pre> <p>there are 3 concrete classes in this hierarchy, for example</p> <pre><code>@Question("openQuestion") @Entity public class OpenQuestion extends QuestionUnit { public void setQuestionContent(String questionContent) { this.questionContent = questionContent; } private String questionContent; public OpenQuestion() { this(null, new OpenQuestionAnswer()); } public OpenQuestion(String questionContent, AnswerUnit correctAnswer) { super(correctAnswer); this.questionContent = questionContent; } public String getQuestionContent() { return this.questionContent; } public String getTitle() { return questionContent; } } @Transactional(propagation = Propagation.REQUIRED) public void update(QuestionList questionList) { entityManager.merge(questionList); } </code></pre> <p>this method is used for all updates, because the list is in the detached state.</p>
 

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