Fetch parent-id in a bidirectional hibernate mapping - json

Question

I have a spring rest application with two entities with a bidirectional relationshop (one-to-many, many to one). To overcome nested fetching issues, #JsonManagedReference/#JsonBackReference has been used for a perent/child relationship between entities.
The entites look as this:
#Entity
#Table(name = "Parent")
#JsonInclude(JsonInclude.Include.NON_NULL)
public class Parent implements java.io.Serializable {
private Integer id;
private List<Child> childList;
#Id
#GeneratedValue(strategy = IDENTITY)
#Column(name = "ID", unique = true, nullable = false)
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
#OneToMany(mappedBy = "parent", fetch = FetchType.LAZY)
#JsonManagedReference
public List<Child> getChildList() {
return childList;
}
public void setChildListe(List<Child> childListe) {
this.childList = childList;
}
}
#Entity
#Table(name = "Child")
#JsonInclude(JsonInclude.Include.NON_NULL)
public class Child implements java.io.Serializable {
private Integer id;
private Parent parent;
#Id
#GeneratedValue(strategy = IDENTITY)
#Column(name = "ID", unique = true, nullable = false)
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "ParentID")
#JsonBackReference
public Parent getParent() {
return parent;
}
public void setParent(Parent parent) {
this.parent = parent;
}
}
This works fine when fetching the Parent element, the childset is then fetched alongside and displayed as an json-array.
However, there is no reference to parent in the child element due to the usage of jsonbackreferance.
I would like to either get the parent object attached to the child-elemens in the json response, or at least fetch the parent id, when requesting for a singlle child element through rest.
All feedback will be appriciated :)

Answer 1

I had a similar issue and I solved it by using custom serializer/deserializer
This is how I procedeed (please adapt it to your own code):
ChildSerializer
public class ChildSerializer extends StdSerializer<Child> {
private static final long serialVersionUID = -265706839304575646L;
public ChildSerializer(Class<Child> t) {
super(t);
}
public ChildSerializer() {
this(null);
}
#Override
public void serialize(Child child, JsonGenerator jg, SerializerProvider sp)
throws IOException, JsonGenerationException {
jg.writeStartObject();
jg.writeStringField("name", child.getName());
jg.writeStringField("surname", child.getSurname());
Parent parent = child.getParent();
jg.writeObjectFieldStart("parent");
jg.writeStringField("name", parent.getName());
jg.writeStringField("surname", parent.getSurname());
jg.writeEndObject();
}
}
Child
#JsonSerialize(using = ChildSerializer.class)
public class Child implements Serializable {
private static final long serialVersionUID = 7902561110976676934L;
private String name;
private String surname;
private Parent parent;
public Child(String name, String surname, Parent parent) {
this(name, surname);
this.parent = parent;
}
public Child(String name, String surname) {
this();
this.name = name;
this.surname = surname;
}
public Child() {
super();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public Parent getParent() {
return parent;
}
public void setParent(Parent parent) {
this.parent = parent;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + ((parent == null) ? 0 : parent.hashCode());
result = prime * result + ((surname == null) ? 0 : surname.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Child other = (Child) obj;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (parent == null) {
if (other.parent != null)
return false;
} else if (!parent.equals(other.parent))
return false;
if (surname == null) {
if (other.surname != null)
return false;
} else if (!surname.equals(other.surname))
return false;
return true;
}
}
Parent
public class Parent implements Serializable {
private static final long serialVersionUID = -5604725691780073247L;
private String name;
private String surname;
private List<Child> childs;
public Parent(String name, String surname) {
this();
this.name = name;
this.surname = surname;
}
public Parent(String name, String surname, List<Child> childs) {
this(name, surname);
this.childs = childs;
}
public Parent() {
super();
childs = new ArrayList<Child>();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public void addChild( Child child )
{
if( !childs.contains(child) )
{
child.setParent(this);
childs.add(child);
}
}
public List<Child> getChilds() {
return childs;
}
public void setChilds(List<Child> childs) {
this.childs = childs;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((childs == null) ? 0 : childs.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + ((surname == null) ? 0 : surname.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Parent other = (Parent) obj;
if (childs == null) {
if (other.childs != null)
return false;
} else if (!childs.equals(other.childs))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (surname == null) {
if (other.surname != null)
return false;
} else if (!surname.equals(other.surname))
return false;
return true;
}
}
My test method:
#Test
public void testJson()
{
try
{
Parent p = new Parent(UUID.randomUUID().toString(), UUID.randomUUID().toString());
for (int i = 0; i < 10; i++) {
p.addChild(new Child(UUID.randomUUID().toString(), UUID.randomUUID().toString()));
}
ObjectMapper om = new ObjectMapper();
String childJson = om.writeValueAsString(p.getChilds().get(0));
System.out.println(childJson);
} catch (Exception e)
{
e.printStackTrace();
}
}
The final output was:
{"name":"b86eab86-9858-4536-9c5c-d44d22810fc1","surname":"9a6249f0-58df-44e5-a1b9-31fbad6e9f49","parent":{"name":"74b0cd97-64a1-4547-ab22-4e4eedd0759b","surname":"a33c79f3-6f26-478b-9e24-7df96b3b1f68"}}
Sadly annotation are powerfull but not always they allow to you to obtain what you need and so you have to do it "by yourself"
I hope this can be usefull