Java.Hibernate.Beginner.Explain mappedBy and joinTables?

🔹 By default in JPA/Hibernate:

• If you write @OneToMany without mappedBy, Hibernate creates a join table to associate the two entities.
• A join table is a separate table just to link parent IDs and child IDs → unnecessary if you already have a foreign key.

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🔹 Example without mappedBy (unidirectional):

@Entity
public class Department {
    @OneToMany // no mappedBy!
    private List<Employee> employees = new ArrayList<>();
}

🔹 What Hibernate does:
Creates an additional table like department_employee with:

department_employee
--------------------
department_id | employee_id

This join table tracks which employees belong to which department, because Hibernate has no idea that the Employee table might have a department_id foreign key.

🔹 Example with mappedBy (bidirectional):

@Entity
public class Department {
    @OneToMany(mappedBy = "department")
    private List<Employee> employees;
}

@Entity
public class Employee {
    @ManyToOne
    @JoinColumn(name = "department_id") // actual foreign key in Employee table
    private Department department;
}

🔹 What Hibernate does now:
✅ No join table needed!
✅ Hibernate sees the department field in Employee as the owning side, which already maps a foreign key (department_id) directly in employees table:

employees
-------------------------
id | name | department_id

🔹 So what does mappedBy do?
✅ Tells Hibernate which field in the child entity owns the relationship.
✅ Signals: “This one-to-many association should rely on the child’s foreign key column, not a separate join table.”

🔹 Key takeaway:
🟢 With mappedBy → Hibernate uses a foreign key in the child table (no join table).
🔴 Without mappedBy → Hibernate creates a separate join table to store parent-child associations.

✅ Bottom line:
The mappedBy attribute in @OneToMany avoids unnecessary join tables by telling Hibernate to rely on the existing foreign key in the “many” side’s table.