How to return an object with embedded objects to a jquery get call?

Question

I use jquery.get() to make a call to the Spring-Boot backend, like this:

    var url = "http://localhost:8080/api/v1/get_holdings?userId=1"; 
    $.get(url, function(payload,status) {       
        if (status == "success") {
            $.each(payload.data, function (key,entry) {
              ...
            })
        }
        else {...

In SpringBoot I create a model that has an embedded model. In this case the model is called Holding and the embedded model is called Account, like this:

@Entity
@Table(name="holding")
public class Holding {
...
    @ManyToOne  //Many holdings to One Account
    @JoinColumn(name="account_id", nullable=false)
    private Account account;
...
//getters and setters...

The call to the database returns the data correctly. Then I try to return it to the jquery ajax call like this:

JsonResponse response = new JsonResponse();
List<Holding> holdings = holdingRepo.getUserHoldings(userId);
response.setData(holdings); //response looks correct but browser shows a 500 error.  Why?
return response;

JsonResponse is just a wrapper that I use for all my responses. I've used it many times and has worked fine for many other calls:

    public class JsonResponse {
        private String status = null;
        private Object data = null;
        //getters and setters...

It returns to the browser ok, but the browser gives a 500 error.

There are two ways that I can get the browser to accept the data.

1. create a separate model that contains just the data I need with no embedded models
2. create the json manually using the toString() method (pain in the butt)

I've tried converting the Holding object to JSON using Gson.toJson and JSONArray(holdings) but both of those methods fail. What am I missing?

Answer 1

You are using a relation. I suppose there happens an exception, like following:

org.hibernate.LazyInitializationException: could not initialize proxy

This can happen because Hibernate tries to optimize performance and does not load objects in the relation.

Then there are many answers to this question on SO.

1. One solution can be to force Hibernate to initialize the proxy via

Hibernate.initialize( entity )

2. Another approach can be not to send entities to the client, but to transform them into data transfer objects. If you do such transformation within the transaction, Hibernate will resolve all needed objects automatically and you will not need any tricks.