can i retrieve child entity containing parent entity id by using Spring Data Jpa one-to-many unidirectional relationship - spring

Question

Here is my parent entity:
#Entity
public class Parent {
#Id
int parentId;
String name;
#OneToMany()
#JoinColumn(name="parent_id")
List<Child> childList;
}
public class Child {
int childId;
//if i am taking this property as non-transient application won't run. but i need parent Id without changing the class structure..
#Transient
int parentId;
// ... some other properties
}
Insertion is successful as two tables are created : parent(id,name),
child(id,name,parent_id).
But when I retrieve the Parent record then in the Child object, the
parentId property remains 0.

Answer 1

i found a way to retrieve the parentid from the child entity by doing a bidirectional mapping of the Parent-child relationship. You can get the ParentId by using a getter method from the child entity that returns the parentId.
Parent Entity
#Entity
#Table(name = "Parent")
public class Parent implements Serializable {
#Column(name = "ID", nullable = false, length = 10)
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private int Id;
#Column(name = "Name")
public String name;
#OneToMany(mappedBy = "parent", fetch = FetchType.LAZY)
private Set<Child> children = new HashSet<>();
public Parent() {
}
//getters and setters omitted for brevity
}
//Child Entity
#Entity
#Table(name = "Child")
public class Child implements Serializable{
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private int Id;
#Column(name = "Name")
private String name;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "parentId", nullable = false)
#OnDelete(action = OnDeleteAction.CASCADE)
private Parent parent;
public Child() {
}
public int getId() {
return Id;
}
public void setId(int id) {
Id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#JsonIgnore
public Parent getParent() {
return parent;
}
//getter method to retrieve the parent id in the child entity
public int getParent_Id(){
return parent.getId();
}
public void setParent(Parent parent) {
this.parent = parent;
}
}
Notice the method getParent_Id() which returns the parent Id, since getter methods are used to return object, the parentId would be returned as part of the child entity anytime it is fetched.
Also note the use of #JsonIgnore on the getParent() method, this to avoid an infinite recursion going on during serialization since Parent refers to Child and Child refer to Parent.