How to privent population of same data in DataBase with Hibernate - database

Question

I'm doing my own project and have something to store in mysql database.
After thinking what should be a best solution DO NOT repeat same value's in DataBase again I didn't find an answer.
So let's example,
Table Cars with rows Cars_ID and Cars_VALUE
-=JoinTable between Cars and Cars_CreationDate=-
Table Cars_CreationDate with rows Cars_CreationDate_ID and Cars_CreationDate_VALUE
-=JoinTable between Cars_CreationDate and Cars_Description=-
Table Cars_Description with rows Cars_Description_ID and Cars_Description_VALUE
In hibernate entities I would have relationships like this:
Between Entity Cars_CreationDate and Cars #ManyToMany, owner of relationships is Cars_CreationDate. Because many Cars can be made in one date and one date can have many cars.
Between Entity Cars_Description and Cars #ManyToOne, owner of relationships is Cars entity, entity Cars_Description belongs to Cars. Entity Cars_Description everytime extracts from database with Cars. fetchType=EAGER
Now let's see if we have in DataBase some Cars made already and I want to add new date to this cars I would say:
Cars cars = (Cars)session.get(Cars.class, int idindatabase); // If we do this for a couple times more for different Cars we will have THE SAME Cars_Description_VALUE corresponds to different Cars so it would looks like this
2013-07-03 to CarOne
2013-07-03 to CarTwo
2013-07-03 to CarTree
and etc. So I don't need this dates repeat every time.
How do not let this problem to be??? Thank you

Answer 1

I don't see much value in using a separate entity/table to store a single value to avoid having to write them out - either a car has a date or a car has a reference to a date. I'd put the date in Car and be done with it. But in theory, what you want is a foreign key in Car to reference your Date. Each car will have a date ID that is used to find the date row/entity. Your date objects then can have a 1:M relationship back, so they can have a collection of cars.
in CarEntity:
#ManyToOne
#JoinColumn(name="CREATIONDATE_ID"
DateEntity creationDate;
In DateEntity:
#OneToMany(mappedby="creationDate")
Collection<CarEntity> cars;