JPA + JSON deserialization - json

Question

I'm quite new to JPA and using Java as backend for REST services, and I'd like to store a JSON into the database, and wanted to check what is the best way to do so. Please let me know in case I'm taking the "long path".
(I'm using Spring)
My data:
{
frequency: "Week"
isGoalOrMore: "true"
name: "Develop"
targetValue: "5"
type: "Average"
}
Habit.java
#Entity
#Table(name = "habits")
public class Habit {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private long id;
private String name;
#ManyToOne(fetch = FetchType.EAGER)
#JoinColumn(name = "id_habit_type")
private HabitType type;
private boolean isGoalOrMore; //basically means, achieve goal or do more
private double targetValue;
#ManyToOne(fetch = FetchType.EAGER)
#JoinColumn(name = "id_frequency")
private Frequency frequency;
//getters and setters
}
HabitType.java
#Entity
#Table(name = "habits_type")
public class HabitType {
#Id
private Long id;
private String description;
}
DB Model (mysql)
habits
--id
--name
--id_frequency
--id_habit_type
habits_type
--id
--description
Problem
When I try to save this data I receive an error as below:
com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class com.tiperapp.model.HabitType] from String value ('Average'); no single-String constructor/factory method
How can I solve it?
By reading some topics, one option would be to write a JSON deserializer. Other cases they solved it by fixing their JPA.
I'd like to know which would be recommend.. can you guys please help me how to deserialize this, the best way ?

Answer 1

The best solution, IMHO, is to think of the data that you send and receive to/from the browser (which I'll call the DTOs), and the data that you store in the database, as two distinct things.
Sure, they often look the same. But they're different. You can't have exactly the same model for both. For example:
the entities constitute a huge graph, often containing bidirectional associations, and which has no limit, since everything can be loaded lazily. For example, an order line has an order, which has a buyer, which has an address, which has a city, which has a country, which has... You can't reasonably serialize all this graph when the browser asks for an order line, otherwise you'll serialize half of the database to JSON.
the JSON sometimes has a different representation of the same thing than the database. For example, you store all the habit types in the database as a row with an ID and a description, but it seems the JSON only really cares about the description. The ID seems to be a way to avoid duplicating the same description everywhere.
many attributes of the entities can not be seen (for security reasons) by the end user, or are only relevant for some use cases, etc.
So I would thus use different classes for both. When you receive a HabitDTO as JSON, you deserialize it with Jackson to a HabitDTO instance, and then find the HabitType entity correspondint to the description in the DTO, and create/update the Habit entity instance based on the corresponding information in the HabitDTO.
To recap: the entities contain the complete business model of your application, used to implement all the functional use cases. The DTOs contain serialized information and are used to transfer a small part of the information to/from the client, often for a specific use case. Having a clear distinction between the two allows much more flexibility: you can change the underlying persistence model without changing the interface of your services, or vice versa.

Answer 2

Your json is wrong.
type is not mapped to a string but to an object. You can do that by using this :
{
frequency: "Week"
isGoalOrMore: "true"
name: "Develop"
targetValue: "5"
type: {
description: "A DESCRIPTION"
id: "Average"
}
}