MVC 5, Entity Framework 6 and Many to Many Relationship

Question

I've two model classes:

public Service()
     public int CodeService { get; set; }
     public string DesignationService { get; set; }
     public virtual ICollection<User> ServiceToUser { get; set; }
public User()
    public int UserID { get; set; }
    public string FirstName { get; set; }
    public virtual ICollection<Service> UserToService { get; set; }
}

The above code generates me 3 tables Service, User & ServiceUsers(autogenerated by EF)

 

i want insert in UserID = 5 and CodeService = 8 in ServiceUsers table.()

 

values 5 already exist in table User

values 8 already exist in table Service

 

i use webapi

var apiurl = "http://localhost:14405/api/ServiceApi";
var data = {
    DesignationService: $("#description").val(),
    serviceToUser: user // usr is list of user with  userid=5
}

$.ajax({
    url: apiurl,
    type: 'POST',
    dataType: 'json',
    data: data,
    success: function (d) {
    noty({ text: 'Association réussie', layout: 'topRight', type: 'success'
    });
},
error: function () {
      noty({ text: 'Association échouée', layout: 'topRight', type: 'error' });
      }
});

public void addService(Service service)
{
    Attach(service);
    bd.Service.Add(service);
    bd.SaveChanges();
}
private void Attach(Service service)
{
    int l = service.ServiceToUser.Count();
    for (int i = 0; i < l; i++)
    {
        bd.User.Attach(service.ServiceToUser.ElementAt<User>(i));
     }
}

but when addService is executed it create a new record in service table value 6 and in ServiceUsers table it create 6 for code service and 5 in userid.

 

can you help me to solve this problem.

 

I want only insert ServiceUsers

 

thanks