使用@OneToMany关系时，即使数据库中只有一个对象，SELECT也会返回多个对象-Java，Hibernate，PostgreSQL

Question

I have two tables in my database: 1. Warehouse 2. WarehouseItem

Relation between them are like listed below:

@Entity
@Table(name = "warehouse")
public class WarehouseModel {

     @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy="warehouse")
     private List<WarehouseItemModel> _items;

and

@Entity
@Table(name = "warehouseItem")
public class WarehouseItemModel {
    @ManyToOne
    public WarehouseModel warehouse;

and now I want to SELECT all the objects of the entity WarehouseModel:

public List getObjects(Class pClass)
    {
        startTime();
        connect();
        Session session = getSession();
        Transaction lTransaction = session.beginTransaction();
        List lRet = session.createCriteria(pClass).list();
        lTransaction.commit();
        endTime("getObjects: " + lRet.size() + " objects");
        Collections.reverse(lRet);
        return lRet;
    }

In my database I have:

• 1x object in the table: Warehouse (WarehouseModel.java)
• 5x objects in the table: WarehouseItem (WarehouseItemModel.java)

When I want to retrive all the Warehouses including related WarehouseItems:

databaseConnector.eDocumentConnector.getObjects(WarehouseModel.class)

the result is:

- 5x the same object of WarehouseModel

It seems that there is dependancy that I always get as much entities of the same WarehouseModel as there is WarehouseItemModels inside field WarehouseModel._items

How to fix it and why it happens? (I have more relations like this one in my project and if it happends here, maybe it happends also in the other places)

Project details: - Java 1.8 - Hibernate 5.0.7 - database: PostgreSQL 9.5.2

Answer 1

通过在休眠查询中使用DISTINCT可以解决此问题：

session.createCriteria(pClass).setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY).list(); 

Answer 2

More importantly, you should understand how a 1-to-many relationship works in SQL.

The raw select statement is going to be something like

SELECT
    .
    .
    <what columns you want>
    .
FROM
    warehouse w
       JOIN warehouseItem wi ON (w.id = wi.warehouse_id)
WHERE
    <whatever you need to filter on>

When you join, you're always going to get the number of rows that are related to the primary table's ID. Since there are 5 rows in the child table, 5 rows are returned, where the warehouse columns are duplicated each time but the warehouseItem columns are unique to each row.