Question

我有一个名为＆＃39; Instructions＆＃39;的实体。有时，每个指令必须在它之前和之后跟踪指令。例如，我有新指令B，它继续从现有指令A，指令B必须知道指令A是前一指令，而指令A也必须知道指令B是其后的指令。并不是每个教学都会在教学之前和之后都有。

如何在JPA（EclipseLink）中实现这一点：[一对一+自引用+双向]关系？

到目前为止（还没有工作）我想出了这个：

mysql db：

CREATE TABLE instructions (
instruction_id int(11) NOT NULL AUTO_INCREMENT,
instruction_title varchar(100) NOT NULL,
instruction_text varchar(999) NOT NULL,
instruction_previous_id int(11) DEFAULT NULL,
PRIMARY KEY (instruction_id),
CONSTRAINT instructions_ibfk_3 
FOREIGN KEY (instruction_previous_id) 
REFERENCES instructions (instruction_id));

实体：

@Entity
@Table(name = "instructions")
public class Instructions implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "instruction_id")
private Integer instructionId;
@Basic(optional = false)
@Column(name = "instruction_title")
private String instructionTitle;
@Basic(optional = false)
@Column(name = "instruction_text")
private String instructionText;

@JoinColumn(name="instruction_previous_id", referencedColumnName = "instruction_id", nullable = true)
@OneToOne(optional = true)
private Instructions instructionPrevious;
@OneToOne(cascade = CascadeType.ALL, mappedBy = "instructionPrevious")
private Collection<Instructions> instructionNextCollection;
// other properties, setter & getter
}

目前创建新教学没有问题，阅读时出错

Instructions instruction = em.find(Instructions.class, instructionId);
instruction.getInstructionNextCollection().size(); //error this line

Local Exception Stack: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): org.eclipse.persistence.exceptions.DatabaseException Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'atiits.instructions_instructions' doesn't exist Error Code: 1146 Call: SELECT t1.instruction_id, t1.instruction_urgent, t1.instruction_uploaded_by, t1.instruction_translate, t1.instruction_title, t1.instruction_type, t1.instruction_translate_received, t1.instruction_is_cancelled, t1.instruction_translate_sent, t1.instruction_had_workorder, t1.instruction_text, t1.instruction_update_date, t1.instruction_update_by, t1.instruction_create_by, t1.instruction_translator, t1.instruction_create_date, t1.instruction_company_id, t1.instruction_previous_id, t1.instruction_status_id FROM instructions_instructions t0, instructions t1 WHERE ((t0.Instructions_instruction_id = ?) AND (t1.instruction_id = t0.instructionNextCollection_instruction_id)) bind => [874] Query: ReadAllQuery(name="instructionNextCollection" referenceClass=Instructions sql="SELECT t1.instruction_id, t1.instruction_urgent, t1.instruction_uploaded_by, t1.instruction_translate, t1.instruction_title, t1.instruction_type, t1.instruction_translate_received, t1.instruction_is_cancelled, t1.instruction_translate_sent, t1.instruction_had_workorder, t1.instruction_text, t1.instruction_update_date, t1.instruction_update_by, t1.instruction_create_by, t1.instruction_translator, t1.instruction_create_date, t1.instruction_company_id, t1.instruction_previous_id, t1.instruction_status_id FROM instructions_instructions t0, instructions t1 WHERE ((t0.Instructions_instruction_id = ?) AND (t1.instruction_id = t0.instructionNextCollection_instruction_id))") at org.eclipse.persistence.exceptions.DatabaseException.sqlException(DatabaseException.java:333) at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.basicExecuteCall(DatabaseAccessor.java:687)

Answer 1

您的示例中存在一些混淆，是否每个指令可以由单个指令或许多指令后跟。

如果它是单一的，那么不要将一个集合用于instructionNext。

如果它很多，那么JPA: How to have one-to-many relation of the same Entity type中的示例代码应该会有所帮助。上一条指令需要@ManyToOne，以下内容需要@OneToMany，而不是@OneToOne。

JPA：一对一+自引用+双向

1 个答案: