无法保存实体“外键约束失败”

Question

I am trying to save an entity to a table which was created using the @JoinTable annotation.

This is the main entity where the table is set:

public class MachineGroup {
    @Id
    @GeneratedValue(strategy= GenerationType.AUTO, generator = "machine_groups_seq")
    @SequenceGenerator(name = "machine_groups_seq", allocationSize = 1, initialValue = 2)
    @Column(name = "id")
    private long id;
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "machine_groups_to_machines",
            joinColumns = @JoinColumn(name = "machine_group_id"),
            inverseJoinColumns = @JoinColumn(name = "machine_id"))
    private Set<Machine> machines;
}

The Entity I try to save:

@Entity
@Table(name="machine_groups_to_machines")
@Getter
@Setter
public class MachineGroupToMachine {
    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name = "id")
    private long id;
    @ManyToOne
    @JoinColumn(name = "machine_id")
    private Machine machine;
    @ManyToOne
    @JoinColumn(name = "machine_group_id")
    private MachineGroup machineGroup;

 public MachineGroupToMachine(Machine machine, MachineGroup machineGroup) {
        this.machine = machine;
        this.machineGroup = machineGroup;
    }
}

Now I am trying to save MachineGroupToMachine via the MachineGroupToMachineRepository directly using this:

 public MachineGroup save(Machine machine, MachineGroup machineGroup){
        Optional<MachineGroupToMachine> omgm = machineGroupToMachineRepository.findMachineGroupToMachineByMachineAndMachineGroup(machine, machineGroup);
        if(omgm.isPresent()){
            String names = MessageFormat.format("Machine {0} and Machine Group {1}", machine.getName(), machineGroup.getName());
            throw new EntityAlreadyExistsException(names);
        }
        return machineGroupToMachineRepository.save(new MachineGroupToMachine(machine, machineGroup)).getMachineGroup();
    }

Hibernate tries to perform this query:

Hibernate: insert into machine_groups_to_machines (machine_id, machine_group_id) values (?, ?)

And this is the exception:

Cannot add or update a child row: a foreign key constraint fails (`test_db`.`machine_groups_to_machines`, CONSTRAINT `FK7g1k7n6hssjrwqadtrx0pc840` FOREIGN KEY (`MACHINE_ID`) REFERENCES `machines` (`id`))

Am I saving the entity in a proper manner? Should it be done from somewhere else perhaps?

Answer 1

What is it that you don't understand? You are referring to a machine in this MachineGroupToMachine entity that does not exist (yet?) on the database. If you want to also persist the machine, then you will have to use @ManyToOne(cascade = PERSIST) on the machine association, but I doubt this is what you generally want. So just persist the machine before you persist the MachineGroupToMachine .