calculated fields in Entity: Autowired is null - spring

Question

I have an Entity
#Entity
#Table(name = "orgtree")
public class OrganizationTree {
#Id
#Column(name="ORGANIZATION_ID")
private String organizationId;
#Column(name="ORGANIZATION_NAME")
private String organizationName;
}
and a repository to provide REST access
#RepositoryRestResource(collectionResourceRel = "organizationTree", path = "organizationTree")
public interface OrganizationTreeRepository extends JpaRepository<OrganizationTree,String> {
#Query
#RestResource(path = "findAll", rel = "findAll")
List<OrganizationTree> findAll();
}
So far so good.
Now I want to add a calculated field to my entity
#Autowired
#Transient
private OrgTreeService orgTreeService;
#JsonSerialize
public Integer getPersonCount() {
return orgTreeService.getPersonCount(organizationId);
}
Here I have several problems:
orgTreeService is null
people say that it's a bad practice to use a
service in an Entity
What is the canonical solution to this problem?

Answer 1

A solution I found (or should I call it a hack) is the following:
I annotate a calculated field with a custom serializer:
#Formula(value = "ORGANIZATION_ID")
#JsonSerialize(using=JsonOrgPersonCountSerializer.class)
private String personCount;
In the serializer I compute the person's count:
public class JsonOrgPersonCountSerializer extends JsonSerializer<String> {
#Override
public void serialize(String source, JsonGenerator gen, SerializerProvider prov) throws IOException, JsonProcessingException {
gen.writeString("" + orgTreeService.getPersonCount(source));
}
}
Another solution would be to use some kind of Data Transfer Object where I can call my service.