我在从SQLite数据库加载具有一对一关系的实体时遇到了问题。
当我使用普通的CriteriaQuery来加载它们时，一切正常。但我在某个地方读到，为了性能，最好在查询中连接数据来源的两个表，这样Hibernate就不会将其拆分为2个查询。
当我尝试创建Fetch时，我遇到了`BasicPathUsageException: Cannot join to attribute of basic type`的错误。
这是我用于创建查询的方法：
```java
	private List<Task> loadTasks() {
		
		try {
			Session session = HibernateUtil.getSessionFactory().getCurrentSession();
			session.beginTransaction();
			
			CriteriaBuilder builder = session.getCriteriaBuilder();
			CriteriaQuery<Task> taskQuery = builder.createQuery(Task.class);
			Root<Task> taskTable = taskQuery.from(Task.class);
			Fetch<Task, TaskType> fetch = taskTable.fetch(TaskType_.ID, JoinType.LEFT);    //<- 异常指向这里
			taskQuery.where(builder.equal(taskTable.get(TaskType_.ID).get("status"), "RECEIVED"));
			
			List<Task> loadedTasks= session.createQuery(taskQuery).getResultList();
			session.getTransaction().commit();
			
			return loadedTasks;
			
		} catch (Exception e) {
			
			e.printStackTrace();
			return null;
		}
	}

@Entity(name = "TaskType")
@Table(name = "task_types")
public class TaskType implements Serializable {
	
	private final SimpleIntegerProperty id = new SimpleIntegerProperty();
	private final SimpleStringProperty name = new SimpleStringProperty();
	
	@Id
	@Column(name = "task_type_id", unique = true)
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	public int getId() {
		return id.get();
	}
	
	public void setId(int id) {
		this.id.set(id);
	}
	
	@Column(name = "task_type_name", unique = true)
	public String getName() {
		return name.get();
	}
	
	public void setName(String name) {
		this.name.set(name);
	}
	
	public SimpleIntegerProperty idProperty() {
		return id;
	}
	
	public SimpleStringProperty nameProperty() {
		return name;
	}

@Entity(name = "Task")
@Table(name = "tasks")
public class Task implements Serializable {
	
	private final SimpleIntegerProperty id = new SimpleIntegerProperty();
	private final SimpleStringProperty name = new SimpleStringProperty();
	private TaskType type;
	
	@Id
	@Column(name = "task_id", unique = true)
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	public int getId() {
		return id.get();
	}
	
	public void setId(int id) {
		this.id.set(id);
	}
	
	@Column(name = "task_name", unique = true)
	public String getName() {
		return name.get();
	}
	
	public void setName(String name) {
		this.name.set(name);
	}
	
	@OneToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "task_type_id")
	public TaskType getType() {
		return type;
	}
	
	public void setType(TaskType type) {
		this.type = type;
	}
	
	public SimpleStringProperty nameProperty() {
		return name;
	}
	
	public SimpleIntegerProperty idProperty() {
		return id;
	}

<details>
<summary>英文:</summary>
I&#39;m having problems with loading entities with a one-to-one relationship from a sqlite database.
When I use a plain CriteriaQuery to load them everything works fine. But I&#39;ve read somwhere that for the performance it&#39;s better to join the two tables the data is coming from in the query so hibernate won&#39;t make 2 queries out of it.
I am getting a `BasicPathUsageException: Cannot join to attribute of basic type` when I am trying to create a Fetch thingy.
 
This is the method I use to create the query:

private List<Task> loadTasks() {
	
	try {
		Session session = HibernateUtil.getSessionFactory().getCurrentSession();
		session.beginTransaction();
		
		CriteriaBuilder builder = session.getCriteriaBuilder();
		CriteriaQuery<Task> taskQuery = builder.createQuery(Task.class);
		Root<Task> taskTable = taskQuery.from(Task.class);
		Fetch<Task, TaskType> fetch = taskTable.fetch(TaskType_.ID, JoinType.LEFT);    //<- exception is pointing here
		taskQuery.where(builder.equal(taskTable.get(TaskType_.ID).get("status"), "RECEIVED"));
		
		List<Task> loadedTasks= session.createQuery(taskQuery).getResultList();
		session.getTransaction().commit();
		
		return loadedTasks;
		
	} catch (Exception e) {
		
		e.printStackTrace();
		return null;
	}
}

This is the TaskType class:

private final SimpleIntegerProperty id = new SimpleIntegerProperty();
private final SimpleStringProperty name = new SimpleStringProperty();
@Id
@Column(name = "task_type_id", unique = true)
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId() {
	return id.get();
}
public void setId(int id) {
	this.id.set(id);
}
@Column(name = "task_type_name", unique = true)
public String getName() {
	return name.get();
}
public void setName(String name) {
	this.name.set(name);
}
public SimpleIntegerProperty idProperty() {
	return id;
}
public SimpleStringProperty nameProperty() {
	return name;
}

And this is the Task class which contains a task type object:

private final SimpleIntegerProperty id = new SimpleIntegerProperty();
private final SimpleStringProperty name = new SimpleStringProperty();
private TaskType type;
@Id
@Column(name = "task_id", unique = true)
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId() {
	return id.get();
}
public void setId(int id) {
	this.id.set(id);
}
@Column(name = "task_name", unique = true)
public String getName() {
	return name.get();
}
public void setName(String name) {
	this.name.set(name);
}
@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "task_type_id")
public TaskType getType() {
	return type;
}
public void setType(TaskType type) {
	this.type = type;
}
public SimpleStringProperty nameProperty() {
	return name;
}
public SimpleIntegerProperty idProperty() {
	return id;
}

How is this fetch() supposed to be used or how can I get the code to load all tasks including their task types? Am I using some annotation wrong?
I tried to look the exception up but I could not find any solution or understand how to apply it to my situation.
Thanks for any help!
</details>
# 答案1
**得分**: 2
`fetch`方法和`join`方法一样，仅适用于关联操作。
您可以使用以下方法替代：
```java
taskTable.fetch(Task_.TYPE, JoinType.LEFT);
taskQuery.where(builder.equal(taskTable.get(Task_.TYPE).get("status"), "RECEIVED"));