HIbernate无法使用外键删除实体。 外键设置为null

Question

This question has been asked in many forms here but none of the solutions seem to work for me. I'm trying to delete the parent entity and I want all of the child entities to also be deleted.

My entities:

@Entity
@Table(name = "item", catalog = "myshchema")
public class Item implements java.io.Serializable {

@JoinColumn(name = "item_id", insertable = false, updatable = false, nullable = false)
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
private Set<ItemCategory> categories;

/* Getters and Setters and other fields*/
}

Table for Item:

CREATE TABLE `item` (
`item_id` int(11) NOT NULL AUTO_INCREMENT,
`store_id` int(11) NOT NULL,
PRIMARY KEY (`item_id`),
UNIQUE KEY `item_id_UNIQUE` (`item_id`),
KEY `FK_ITEM_STORE_ID_idx` (`store_id`),
CONSTRAINT `FK_ITEM_STORE_ID` FOREIGN KEY (`store_id`) REFERENCES `store`   (`store_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=84 DEFAULT CHARSET=utf8;

And my other entity

@Entity
@Table(name = "item_category", catalog = "myschema")
@IdClass(ItemCategoryIndex.class)
public class ItemCategory implements java.io.Serializable {

@Id
@Column(name = "category_id", unique = true, nullable = false, insertable = false, updatable = false)
private Integer categoryId;
@Id
private Store store;
@Id
private Item item;
@Id
private String categoryName;

/* Getters and Setters */
}

Table for ItemCategory:

CREATE TABLE `item_category` (
`category_id` int(11) NOT NULL AUTO_INCREMENT,
`store_id` int(11) NOT NULL,
`item_id` int(11) NOT NULL,
`category_name` varchar(45) NOT NULL,
PRIMARY KEY (`category_id`),
UNIQUE KEY `category_id_UNIQUE` (`category_id`),
UNIQUE KEY `IDX_UNIQUE_STORE_CATEGORY`     (`store_id`,`item_id`,`category_name`) USING BTREE,
KEY `FK_CATEGORY_STORE_ID_idx` (`store_id`),
KEY `FK_ITEM_CATEGORY_ID_idx` (`item_id`),
CONSTRAINT `FK_CATEGORY_STORE_ID` FOREIGN KEY (`store_id`) REFERENCES     `store` (`store_id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_ITEM_CATEGORY_ID` FOREIGN KEY (`item_id`) REFERENCES `item`     (`item_id`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=162 DEFAULT CHARSET=utf8;

I try to delete the item like this:

Item item = entityManager.find(Item.class, idList.get(i));
entityManager.remove(item);

My logs show that Hibernate is trying to set the primary key for ItemCategory to null:

Hibernate: update myschema.item_category set item_id=null where item_id=?
ERROR o.h.e.jdbc.spi.SqlExceptionHelper.logExceptions 146 - Column 'item_id' cannot be null

I even tried looping through the child records and deleting them manually, but Hibernate still issues this update to null query. What am I doing wrong?

Answer 1

I have to break your problem down to two parts

First - let's talk about your database schema design.

According to your schema, item and item_category has a one-to-many relationship meaning an item can have/be-assigned-to different categories but different items cannot have/be-assigned-to the same category.

That is totally fine if it is indeed your business requirement , I mention it because it does not make sense to me and this circumstance rarely happens.

If what you want is that a category can have multiple items and vice versa, item and item_category must be a many-to-many relationship. There should be a join table additionally.

Second - let's say the schema don't change

ItemCategory is the owner of the relationship because it has a foreign key item_id refering to item table. So the ItemCategoy should look roughly like this:

@Entity
@Table(name = "item_category")
public class ItemCategory {

@Id
private Integer categoryId;

private Store store;

@ManyToOne
@JoinColumn(name="item_id", /*cascade = ...*/)
private Item item;

private String categoryName;

/* Getters and Setters */
}

Your Item entity will be roughly like this:

@Entity
@Table(name = "item", catalog = "myshchema")
public class Item implements java.io.Serializable {

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true, mappedBy="item")
private Set<ItemCategory> categories; //`mappedBy`used here because this entity is not the owner of the relationship according to what mentioned above

/* Getters and Setters and other fields*/
}  

To remove all the child entities( ItemCategory ) from Item , simply

em.remove(item);

The orphanRemoval is true , deleting the parent, the children will be deleted as well.

Answer 2

In Hibernate, you need to decide who is owning the relationship. If you have the parent side (ItemCategory) owning the relationship, you will find insertion/deletion of Item+ ItemCategory will involve update of item_id in ItemCategory table (which is what I observed from your exception). In most case it is not preferable. We usually let the children own the relationship. This is done by using mappedBy

(pseudo-code)

class Item {
  //...

  @OneToMany(mappedBy = "item", cascade=ALL, orphanRemoval=true)
  private Set<ItemCategory> categories;
}

class ItemCategory {
  //...

  @ManyToOne
  @JoinColumn(name="item_id")
  Item item;
}

The trick here is mappedBy