#OneToMany Column not showing? - spring

Question

I am new to Hibernate and curious, why my column is not showing.
So I have following code (in kotlin):
#Entity
class Project (
var guid: String = "",
#OneToMany(mappedBy = "project", cascade = [CascadeType.ALL])
var tickets: List<Ticket?>,
var current: Date
){
#Id #GeneratedValue(strategy = GenerationType.AUTO)
var id: Long = 0
}
and the other entity:
#Entity
class Ticket (
var guid: String = "",
#ManyToOne
#JoinColumn(name="project_id")
var project: Project?,
var current: Date
){
#Id #GeneratedValue(strategy = GenerationType.AUTO)
var id: Long = 0
}
In my database, the table "ticket" has a column project_id, with null values.
BUT my other table "Project" has no column for the list of Tickets. What went wrong? Every help will be appreciated, please dont forget to explain.
Thank you!

Answer 1

The reason your Project table has no column for the list of tickets, is that relational databases are designed to have one value per cell. You cannot store a list or array of something in a table. The solution for this is to create another table (let's call it the child table) that references this table (the parent table). Each row in the child table will be one entry of that list, and stores a reference to the parent list. So in a way, the relationship has been inverted: instead of storing the tickets in the Project table, a reference to the Project table is stored in the Ticket table.
In your case, you're indicating that the project_id should be used as a reference, since you're mentioning it in the #JoinColumn. However, there is no such property in the Project table, so you should add it. Hibernate is expecting that this is the column to link the two tables.
Also: don't forget to create a foreign key constraint to the parent table on the column that you're using in the Ticket table either.