具有雙向相同實體關系的Hibernate N Select問題

Question

我有以下屬性的Item.java實體

@OneToMany(cascade=CascadeType.ALL, mappedBy="kitItem", fetch=FetchType.EAGER) 
private Set<KitItemDetails> kitItemDetails = new HashSet<KitItemDetails>(0);

.... Other properties with getter and setter

我有帶有以下屬性的KitItemDetails.java實體

@ManyToOne(cascade=CascadeType.ALL)  
private Item kitItem;

在我的DAO訪問類中，我們使用以下查詢以EAGER提取具有其他相關屬性的所有項目

List<Item> items = getCurrentSession().createCriteria(Item.class).addOrder(Order.asc("itemOrder"))
                .list();

但是為此，它觸發n個選擇查詢來影響項目詳細信息，如下所示

select kititemdet0_.kitItem_id as kitItem_2_9_0_, kititemdet0_.id as id1_12_0_, kititemdet0_.id as id1_12_1_, kititemdet0_.kitItem_id as kitItem_2_12_1_, kititemdet0_.ITEM_ID as ITEM_ID3_12_1_, item1_.id as id1_9_2_, item1_.BAR_CODE as BAR_CODE2_9_2_, item1_.COLOR_CODE as COLOR_CO3_9_2_, item1_.IS_ACTIVE as IS_ACTIV4_9_2_, item1_.ITEM_CATEGORY_ID as ITEM_CA14_9_2_, item1_.ITEM_CODE as ITEM_COD5_9_2_, item1_.ITEM_DISP_NAME as ITEM_DIS6_9_2_, item1_.ITEM_IMAGE as ITEM_IMA7_9_2_, item1_.ITEM_NAME as ITEM_NAM8_9_2_, item1_.ITEM_ORDER as ITEM_ORD9_9_2_, item1_.ITEM_PRICE as ITEM_PR10_9_2_, item1_.ITEM_PRICE_WITH_TAX as ITEM_PR11_9_2_, item1_.ITEM_SUB_CATEGORY_ID as ITEM_SU15_9_2_, item1_.ITEM_TYPE as ITEM_TY12_9_2_, item1_.TAX_ID as TAX_ID16_9_2_, item1_.TAX_CODE as TAX_COD13_9_2_, itemcatego2_.id as id1_10_3_, itemcatego2_.COLOR_CODE as COLOR_CO2_10_3_, itemcatego2_.IS_ACTIVE as IS_ACTIV3_10_3_, itemcatego2_.ITEM_CATEGORY_NAME as ITEM_CAT4_10_3_, itemcatego2_.LOCATION_ID as LOCATION6_10_3_, itemcatego2_.PRINTER_NAME as PRINTER_5_10_3_, itemsubcat3_.id as id1_11_4_, itemsubcat3_.COLOR_CODE as COLOR_CO2_11_4_, itemsubcat3_.IS_ACTIVE as IS_ACTIV3_11_4_, itemsubcat3_.ITEM_CATEGORY_ID as ITEM_CAT5_11_4_, itemsubcat3_.ITEM_SUB_CATEGORY_NAME as ITEM_SUB4_11_4_, tax4_.id as id1_21_5_, tax4_.IS_ACTIVE as IS_ACTIV2_21_5_, tax4_.LOCATION_ID as LOCATION6_21_5_, tax4_.TAX_CODE as TAX_CODE3_21_5_, tax4_.TAX_NAME as TAX_NAME4_21_5_, tax4_.TAX_PER as TAX_PER5_21_5_ from KIT_ITEM_DETAILS kititemdet0_ inner join ITEM item1_ on kititemdet0_.ITEM_ID=item1_.id left outer join ITEM_CATEGORY itemcatego2_ on item1_.ITEM_CATEGORY_ID=itemcatego2_.id left outer join ITEM_SUB_CATEGORY itemsubcat3_ on item1_.ITEM_SUB_CATEGORY_ID=itemsubcat3_.id left outer join TAX tax4_ on item1_.TAX_ID=tax4_.id where kititemdet0_.kitItem_id=?

我嘗試了mapdBy，JoinColumn，JoinType，FetchMode等，但沒有解決，可能是什么問題？

Answer 1

通常，將關系設置為EAGER確實會在使用條件查詢時導致“ N個選擇查詢”問題。 您可以通過明確指示Hibernate執行訪存聯接來克服這一問題。 有關更多詳細信息，請參見Hibernate文檔 。

該文檔包含一個與您的場景非常相似的示例（已調整為使用您的類名）：

CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<Item> query = builder.createQuery( Item.class );
Root<Item> root = query.from( Item.class );
root.fetch( "kitItemDetails", JoinType.LEFT);
query.select(root).where(
   builder.and(
     builder.equal(root.get("foo"), foo),
     builder.equal(root.get("bar"), bar)
  )
);
Item item = entityManager.createQuery( query ).getSingleResult();
// Alternatively, .getResultList() to return List<Item>

@Fetch(FetchMode.JOIN)在您的情況下不起作用，因為隨着文檔繼續在其示例中說明FetchMode.JOIN ，請執行以下FetchMode.JOIN ：

之所以不使用JPQL查詢來獲取多個Department實體，是因為FetchMode.JOIN策略將被查詢獲取指令所覆蓋。