如何保存包含实体的新学员，但不创建这些实体，因为它们已经存在于数据库中？

Question

我想保存一个学生，一个学生有一个TargetAudience对象作为属性。这些目标受众都已经在我的数据库中硬编码好了。（targetaudience=校园专业）。现在当我这样发帖时：

{
    "user": {
        "userName": "jan",
        "password": "tibo123",
        "role": "ROLE_STUDENT"
    },
    "targetAudience": {
        "majorCode": "IW E-ICT",
        "campus": {
            "name": "GroepT",
            "street": "Andreas Vesaliusstraat",
            "postalCode": "3000",
            "streetNr": "13"
        }
    }
}

它不起作用，因为每次它为校园创建一个新对象，并且因为我使用名称作为主键，它都会抛出一个异常。spring data jpa不应该查看实体allready是否存在，然后使用它吗？或者我该怎么做？

抱歉我没说清楚这是我第一次发帖

大学生爪哇：

package com.bachproject.demo.student;

import com.bachproject.demo.onderwerp.Onderwerp;
import com.bachproject.demo.targetAudience.TargetAudience;
import com.bachproject.demo.user.User;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import javax.persistence.*;
import java.util.List;

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {

    @Id
    @SequenceGenerator(
            name = "student_sequence",
            sequenceName = "student_sequence",
            allocationSize = 1
    )
    @GeneratedValue(
            strategy = GenerationType.SEQUENCE,
            generator = "student_sequence"
    )
    private Long studentId;

    @OneToOne(
            cascade = CascadeType.ALL,
            fetch = FetchType.EAGER,
            optional = true
    )
    @JoinColumn(
            name = "user_id",
            referencedColumnName = "userId"
    )
    private User user;

    @OneToOne(
            cascade = CascadeType.ALL,
            fetch = FetchType.EAGER,
            optional = true
    )
    @JoinColumn(
            name = "target_audience",
            referencedColumnName = "TargetAudienceId"
    )
    private TargetAudience targetAudience;

    //private List<Onderwerp> preferences;

}

学生管理员。JAVA

package com.bachproject.demo.student;

import com.bachproject.demo.onderwerp.Onderwerp;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/students")
public class StudentController {

    @Autowired
    private StudentService studentService;

    @GetMapping
    @CrossOrigin(origins = "*")
    public List<Student> getStudents(){
        return studentService.getStudents();
    }

    @PostMapping("/register")
    @CrossOrigin(origins = "*")
    public Student registerStudent(@RequestBody Student student) {
        System.out.println(student);
        return studentService.registerStudent(student);
    }
}

学生ervice.java：

package com.bachproject.demo.student;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class StudentService {

    @Autowired
    private StudentRepository studentRepository;

    public List<Student> getStudents() {
        return studentRepository.findAll();
    }

    public Student registerStudent(Student student) {
        return studentRepository.save(student);
    }
}

目标观众。爪哇：

package com.bachproject.demo.targetAudience;

import com.bachproject.demo.campus.Campus;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import javax.persistence.*;

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class TargetAudience {

    @Id
    @SequenceGenerator(
            name = "targetAudience_sequence",
            sequenceName = "targetAudience_sequence",
            allocationSize = 1
    )
    @GeneratedValue(
            strategy = GenerationType.SEQUENCE,
            generator = "targetAudience_sequence"
    )
    private Long TargetAudienceId;

    // for example IW E-ICT-> industriele wetenschappen Elektronica ICT
    private String majorCode;

    @OneToOne(
            cascade = CascadeType.ALL,
            fetch = FetchType.EAGER,
            optional = true
    )
    @JoinColumn(
            name = "campus",
            referencedColumnName = "name"
    )
    private Campus campus;

}

Campus.java：

package com.bachproject.demo.campus;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Campus {

    @Id
    private String name;

    private String street;
    private String postalCode;
    private String streetNr;
}

Answer 1

您在试图保存时遇到了什么异常？堆栈跟踪会更有帮助。然而，你可以通过两种方式来解决这个问题

或者，如果您想完全避免保存TargetAudience，您可以在@瞬态注释中注释TargetAudience属性，这样，它就不会被考虑用于数据库保存。

或者，JPA每次都试图用新的主键保存该对象，因为该对象处于分离状态（如果您的JPA提供程序是hibernate，这是默认的JPA提供程序）。您应该修改StudentService中的registerStudent（）方法，如-

public Student registerStudent(Student student) {
    TargetAudience targetAudience = targetAudienceRepository.findByMajorCode(student.getTargetAudience().getMajorCode());
    student.setTargetAudience(targetAudience);
    return student;
}

因此，您从数据库中显式地获取TargetAudience，并将其设置为您要获取的学生对象。这样，获取的目标受众将处于持久状态（而不是分离状态），并且它不会再次尝试分配新的主键。