Save object in database if it does not already exist (Hibernate & Spring)

Question

I'm working on a Hibernate/Spring application to manage some movies.

The class movie has a many to many relationship with the class genre. Both of these classes have generated id's using the GeneratedValue annotation.

The genre is saved through the movie object by using @Cascade(CascadeType.SAVE_UPDATE) I have placed a unique constraint on the genre's type attribute (which is it's name; "Fantasy" for example).

What I would like to do now is have Hibernate check if there is already a genre with type "Fantasy" and if there is, use that genre's id instead of trying to insert a new record. (The latter would obviously throw an error)

Finally what I need is something like select-before-update but more like select-before-save.

Is there such a function in Hibernate?

Some code:

Movie class

@Entity
public class Movie {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;

private String name;

@Lob
private String description;

@Temporal(TemporalType.TIME)
private Date releaseDate;

@ManyToMany
@Cascade(CascadeType.SAVE_UPDATE)
private Set<Genre> genres = new HashSet<Genre>();

.... //other methods

Genre class

@Entity
public class Genre {

@Column(unique=true)
private String type;

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id

....//other methods

Answer 1

You may be over-thinking this. Any select-before-update/select-before-save option is going to result in 2 DB round trips, the first for the select, and the second for the insert if necessary.

If you know you won't have a lot of genres from the outset, you do have a couple of options for doing this in 1 RT most of the time:

1. The Hibernate second-level cache can hold many if not all of your Genres, resulting in a simple hashtable lookup (assuming a single node) when you check for existence.
2. You can assume all of your genres are already existing, use session.load(), and handle the new insert as a result of the row not found exception that gets thrown when you reference a genre that doesn't already exist.

Realistically, though, unless you're talking about a LOT of transactions, a simple pre-query before save/update is not going to kill your performance.

Answer 2

I haven't heard of such a function in Hibernate select-before-update/select-before-save In situations like these you should treat Hibernate as if it was JDBC.

First if you want to know if you even have such a Genre you should query for it.

if you do. then the SAVE_UPDATE will not create a new one when you add it to a movie. if you don't, Hibernate will create a new Genre row in the database and add the connection to the many_to_many table for you.