防止NonUniqueObjectException休眠

Question

當我試圖給定的圖像添加到相同的目的地城市的兩倍，我得到以下異常：

NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [org.wah.model.ImageEntity#7] 

這是好事，因爲我不想重複。我試圖防止這種情況的發生代碼：

public void addImageToDestination(int idDestination, String imageFileName){ 
    Destination destination = (Destination) getEntity(idDestination); 

    ImageEntityDAO imageDao = new ImageEntityDAO(); 
    ImageEntity image = imageDao.getImage(imageFileName); 

    if(image == null) 
     image = new ImageEntity(imageFileName); 
    else if(destination.getImages().contains(image)){ 
     return; 
    } 

    session.beginTransaction(); 
     destination.getImages().add(image); 
    session.getTransaction().commit(); 
} 

的else-if結構試圖確定它是否已經存在，則無能爲力，回到原樣。但是，else-if條件從不計算爲TRUE並且會話代碼運行，產生NonUniqueObjectException。

我該如何防止這種情況發生？

Answer 1

如果你的意思是destination.getImages().contains(image)從不計算true，應該如果兩個ImageEntities具有相同的城市，你應該重寫ImageEntity的equals - 方法：

@Override 
public boolean equals(Object o) { 
    if (this == o) return true; 
    if (o == null || getClass() != o.getClass()) return false; 

    ImageEntity that = (ImageEntity) o; 

    if (city != null ? !city.equals(that.city) : that.city != null) return false; 

    return true; 
}