Hibernate试图将重复的行插入数据库

Question

我正在尝试使用hibernate建模以下关系： ProcessedUrl有一组句子。每个句子都有对拥有它的URL的引用，它还包含一个单词列表。每个Word都有一组包含它的句子。这是我写的代码：

@Entity
public class ProcessedUrl {
    @Id @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Type(type = "text")
    private String url;

    private Date date;

    @OneToMany(mappedBy = "processedUrl")
    @Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
    private Set<Sentence> sentences = new HashSet<>();

    public ProcessedUrl() {}

    public ProcessedUrl(String url, Date date) {
        this.url = url;
        this.date = date;
    }

    // getters, setters
}


@Entity
public class Sentence {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @ManyToMany
    @OrderColumn(name="word_index")
    @JoinTable(name = "word_sentence",
            joinColumns = {@JoinColumn(name = "sentence_id")},
            inverseJoinColumns = {@JoinColumn(name="word_id")})
    @Cascade({CascadeType.MERGE, CascadeType.SAVE_UPDATE})
    private List<Word> words = new ArrayList<>();

    @ManyToOne
    @JoinColumn(name = "urlId")
    private ProcessedUrl processedUrl;

    public Sentence() {}        
    // getters, setters
}

@Entity
public class Word {
    @Id
    @Column(length = 255)
    private String content;

    @ManyToMany(mappedBy = "words")
    private Set<Sentence> sentences = new HashSet<>();

    public Word() {}
    public Word(String content) {
        this.content = content;
    }
    // getters, setters
}

当我尝试插入新的ProcessedUrl及其内容

时会出现问题

public void index() {
    Session session = HibernateUtils.getSession();
    Transaction transaction = session.beginTransaction();

    ProcessedUrl processedUrl = new ProcessedUrl(url, new Date());

    List<String> sentences = getSentences();
    sentences.forEach(sentenceContent -> {
        Sentence sentence = new Sentence();
        List<Word> words = wordsFromSentence(sentenceContent);
        sentence.getWords().addAll(words);
        sentence.setProcessedUrl(processedUrl);
        words.forEach(w -> w.getSentences().add(sentence));
        processedUrl.getSentences().add(sentence);
    });

    session.merge(processedUrl);

    transaction.commit();
    session.close();
}

private List<Word> wordsFromSentence(String sentenceContent) {
    return Arrays.stream(sentenceContent.split("\\s+"))
            .map(word -> word.replaceAll("[^\\w]", ""))
            .filter(s -> !s.isEmpty())
            .map(Word::new)
            .collect(Collectors.toList());
}

此代码在数据库中给出了错误 ERROR: Duplicate entry 'google' for key 'PRIMARY' 我的问题是如何更改我的代码，以便hibernate不会尝试多次将相同的单词插入数据库。