Entity Framework n to n relationship add object to another object

Question

I have a Model with Clients:

public class Client

{
    [Key]
    public int id { get; set; }
    [Required]
    public string? Hostname { get; set; }
    public ICollection<Software>? Softwares { get; set; }

}

And a Model with Software:

public class Software
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int id { get; set; }
        public string Name { get; set; }
        public ICollection<Client>? Clients { get; set; }
    }

This is supposed to be an n to n connection. How can I add software to my clients?

What I've tried:

public async void add(Software software)
        {
            using (var repo = new ClientRepository(contextFactory.CreateDbContext()))
            {
                client.Softwares.Add(software);
                await repo.Save(client);
            }

Repository:

public async Task Save(Client client)
        {
            _context.Clients.Update(client);
            _context.SaveChanges();
        }
        }

This works for the first software I add, but gives me the following error if I try to add a second one:

SqlException: Violation of PRIMARY KEY constraint 'PK_ClientSoftware'. Cannot insert duplicate key in object 'dbo.ClientSoftware'. The duplicate key value is (7003, 5002).

Answer 1

Q: Do you make a new one with new() every time?

A: no, its a software that already exists

Then that is the error.
The first SaveChanges() will start tracking it as Existing. You can check: The Id will be != 0.

When you then later add it to the same Client you'll get the duplicate error.

So somewher in your code:

  await add(software);       // existing
  software = new Software(); // add this 

Related, change:

public async void add(Software software)

to

public async Task Add(Software software)

Always avoid async void.