why hibernate not set foreign key in the owner side of OneToMany

Question

I have 2 Entities, Hospital and Department, Department reference its hospital by hospital_id in it.

@Entity
public class Hospital {
    ...
    private Set<Department> departments;

    @OneToMany(mappedBy = "hospital", cascade = CascadeType.ALL)
    public Set<Department> getDepartments() {
        return departments;
    }
}

@Entity
public class Department {
    ...
    private Hospital hospital;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "hospital_id")
    public Hospital getHospital() {
        return hospital;
    }
}

I use Rest API to create instance of Entities:

@RequestMapping("/create")
@ResponseBody Hospital create(@RequestBody Hospital hospital){
    hospital = hospitalService.save(hospital);

    return hospital;
}

I post json payload to create a Hospital:

{"name":"t-hospital", "departments":[{"name": "department1"}]}

Since I use SpringBoot, Jackson will auto parse the json payload into Java Object, here its Hospital and Departments in it.

While after I save the hospital, I found the foreign key: 'hospital_id' in department is not set.

Why is this and if possible, how can I make the foreign key set when I save the hospital object?

Answer 1

You should always create an association in owner side before persisting an entity because owner side is responsible for creating relationships.

Modify your code as below (Contains Java 8 Code):

@RequestMapping("/create")
@ResponseBody Hospital create(@RequestBody Hospital hospital){

    hospital.getDepartments().forEach(department->department.setHospital(hospital));
    hospital = hospitalService.save(hospital);

    return hospital;
}

You may face a new problem when converting entity to json because Jackson serializes related entities recursively and you'll get StackOverflow Error. To resolve this issue add @JsonManagedReference along with @OneToMany annotation and @JsonBackReference along with @ManyToOne annotation.

Note:@JsonManagedReference and @JsonBackReference available in new version of Jackson