Using Projecions to fetch a particular column from child table

问题

I have two tables

Loan (id, amount, duration)

LoanStatus(id, status, loan_id)   // just an example, but it has lot more fields in this table

Loan.java

public class Loan{

    private int id;
    private int amount;
    private int duration;
    private LoanStatus loanStatus;

    // getter and setters

}

LoanStatus.java

public class LoanStatus{   // just an example, but it has many fields than what actually given

    private int id;
    private String status;
    private Loan loan;

    //getter and setters

}

Now I would like to get only amount , duration , loanStatus.status using Projections. I've used createAlias() to successfully fetch that particular column, but the problem occurs when setting it to a setter.

Criteria criteria = getSession().createCriteria(Loan.class,"loan");
    criteria.createAlias("loan.loanStatus", "loanStatus")
            .setProjection(Projections.projectionList().add(Projections.property("id"),"id")
            .add(Projections.property("amount"),"amount")
            .add(Projections.property("duration"),"duration")
            .add(Projections.property("loanStatus.status"), "loanStatus"))
            .add(Restrictions.eq("id", id))
            .setResultTransformer(Transformers.aliasToBean(Loan.class));
    return criteria.list();

I've an error which as follows.

IllegalArgumentException occurred while calling setter for property [com.site.dto.Loan.loanStatus (expected type = com.site.dto.LoanStatus)]; target = [com.site.dto.Loan@4083974a], property value = [Pending]

So I'm getting my expected column value "Pending", but the problem is when setting it to a setter. I've seen many question for projections, but most of them was based on Restrictions using Projections but not fetching a child's particular column using projections.

回答1:

Write your own custom transformer. The following implementation might be exactly what you need

https://github.com/samiandoni/AliasToBeanNestedResultTransformer . The usage example as written in their docs

class Person {
  private Long id;
  private String name;
  private Car car;
  // getters and setters
}

class Car {
  private Long id;
  private String color;
  // getters and setters
}

List<Person> getPeople() {
  ProjectionList projections = Projections.projectionList()
    .add(Projections.id().as("id"))
    .add(Projections.property("name").as("name"))
    .add(Projections.property("c.id").as("car.id"))
    .add(Projections.property("c.color").as("car.color"));

  Criteria criteria = getCurrentSession().createCriteria(Person.class)
    .createAlias("car", "c")
    .setProjection(projections)
    .setResultTransformer(new AliasToBeanNestedResultTransformer(Person.class));

  return (List<Person>) criteria.list();
}

// each car of Person will be populated

来源：https://stackoverflow.com/questions/28796400/using-projecions-to-fetch-a-particular-column-from-child-table

标签

java

mysql

hibernate

projection