public class Invoice {
 
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;
 
    // Some fields ...
    
 
    @ToString.Exclude
    @JsonIgnore
    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(
            name = "invoice_elements",
            joinColumns = @JoinColumn(name = "invoice_id"),
            inverseJoinColumns = @JoinColumn(name = "element_id")
    )
    private List<CostElements> costElements;
}

public class CostElements {
 
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;
 
    @Column(name = "name")
    private String name;
 
    @ToString.Exclude
    @JsonIgnore
    @ManyToMany(mappedBy = "costElements")
    private List<Invoice> invoices;
}

 // InvoiceRequest Dto has both invoice data and cost elements data.              

@Override
        public void saveInvoice(InvoiceRequest invoiceRequest) {
    
        CostElements costElement1 = new CostElements();
        costElement1.setName(invoiceRequest.getName());
        costElement1.setInvoices();
 
        CostElements costElement2 = new CostElements();
        costElements2.setName(invoiceRequest.getName());
        costElements2.setInvoices();
 
        List<CostElements> elementList = new ArrayList<>();
        elementList.add(costElement1);
        elementList.add(costElement2);
 
        Invoice invoice = new Invoice();   
        invoice.setCostElements(elementList);
}

Look at the saveInvoice() method. My question is how should I save costElements and Invoice correctly. How can I handle these setter methods correctly ? -> costElement1.setInvoices(); costElements2.setInvoices(); Because even though there is a setter method called -> setInvoices(); But we have only one Invoice. Can anyone please clarify this problem for me ?

I tried this;

for (CostElements costElement : elementList) {
     costElement.getInvoices().add(invoice);
}

But it is not working as I expected.