How should i define which Hibernate mapping to use, and when/where to use it? - spring-boot

Question

i and having a use case where there are 2 kinds of users namely the "clients" and "professionals". These 2 entities have a parent entity called the "users" where each "user" has one entry in either "client/professional" depending on their role.
Let's consider a "clients".
A "user" has a one-to-one mapping with a "client"
A client might have several "companies" under him, i.e "client" has one-to-many relationship with "companies".
I am creating a REST API for this use case using spring boot. I still dont have any idea about why i should be using mapping in Hibernate. So far the only advantage i see is that, the CASCADING property of it. If a "user" gets removed, all the tables having the "user-id" will also be flushed. But consider a scenario where i need to add "companies" for a "client". I am confused to whether i should persist "companies" via "clients" entity or should i directly persist to "client" entity. I dont see any major advantage here because in both cases we are checking whether a "client" exists with the given ID before persisting in the "clients" table.
User Entity
public class User {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long UID;
private Integer userRoleId;
private String username;
private String email;
private String phoneNumber;
private String firstName;
private String lastName;
private Long dateOfJoin;
private Boolean activeStatus;
private Long createdAt;
private Long updatedAt;
#OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "user")
private Client client;
}
Client Entity
public class Client {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long CID;
#Column(unique = true)
private Long userId;
private Long createdAt;
private Long updatedAt;
#OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "client")
private ClientCompany clientCompany;
#OneToOne(cascade = CascadeType.ALL)
#JoinColumn(name = "userId" ,referencedColumnName = "UID", insertable = false, updatable = false)
private User user;
}
Client Company Entity
public class ClientCompany {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long CCID;
private Long clientId;
private String email;
private String phoneNumber;
public String streetAddress1;
public String streetAddress2;
public String zipCode;
public String city;
public String state;
public String country;
private Long createdAt;
private Long updatedAt;
#OneToOne(cascade = CascadeType.ALL)
#JoinColumn(name = "clientId", referencedColumnName = "CID", insertable = false, updatable = false)
private Client client;
}

Answer 1

The advantage of using Hibernate/JPA is that you do not need to code JDBC calls.
You just use objects.
In your scenario,
load a Client instance from the database;
create a ClientCompany object;
assign the Client instance to it (no need to check the client existence since you loaded it from the database);
save to database.
Hibernate will take care of everything without you writing any SQL statements.
Step 1) can also be replaced with creating a new Client that will be saved to the database, but again Hibernate will handle saving correctly (if you configured it correctly).