将数据从视图模型添加到模型

Question

So here is my controller. The ViewModel contains a member's PIN and an Events ID. Now I want to find which event is associated with the Id, and which member is associated with the Pin. Once I have that information, I want to associate this event to the members whose PIN was entered. I'll add the member class and event class below. (it is a many to many relationships) I'm just confused on how to add the event to the member table

[HttpPost]
public ActionResult SignUp(MemberEventViewModel Vmodel) //signup action
{
var MemberinDB = _context.Members.Where(c => c.PIN == Vmodel.Member.PIN);

var EventinDB = _context.Events.Where(c => c.Id == Vmodel.Events.Id);
_context.Members.Add(EventinDB);
_context.SaveChanges();
}

member class

public class Member
{
    public int Id { get; set; }
    [Required]
    [MaxLength(4, ErrorMessage = "PIN must be 4 numbers long"), MinLength(4, ErrorMessage = "PIN must be 4 numbers long")]
    public string PIN { get; set; }
    [Required]
    [Display(Name ="First Name")]
    public string FirstName { get; set; }
    [Required]
    [Display(Name = "Last Name")]
    public string LastName { get; set; }
    [Display(Name = "Date of Birth")]
    public DateTime? Birthdate { get; set; }
    public virtual ICollection<Event> Events { get; set; }

}

event class

public class Event
{

    public int Id { get; set; }
    [Required]
    public string Name { get; set; }
    [Required]
    [Display(Name = "Date")]
    public DateTime? EventDate { get; set; }
    [Required]
    [Display(Name = "Start Time")]
    public TimeSpan EventStartTime { get; set; }
    [Required]
    [Display(Name = "End Time")]
    public TimeSpan EventEndTime { get; set; }
    public int EventTotalTime{ get; set; }
    public virtual ICollection<Member> Members { get; set; }

}

Answer 1

Use the navigation properties to associate your new relationship.

var MemberinDB = _context.Members.Include(c => c.Events).Single(c => c.PIN == Vmodel.Member.PIN);

if(MemberinDB.Events.Any(e => x.Id == VModel.Event.Id))
    return; // Member and event already associated.

var EventinDB = _context.Events.Single(c => c.Id == Vmodel.Event.Id);
MemberInDB.Events.Add(EventinDB);

_context.SaveChanges();

This loads the member and includes the events associated. Your member and events relationship is many to many, so you could do this from the member to add the event or from the event to add the member. The above example checks that the event hadn't been added, then retrieves it and adds it to the member if not.

Your view model implies that a member has Events, rather than passing a view model to represent just a new member-event association. If you are passing a view model that contains a member and all of their events (including existing associations and new ones) then you may need some additional logic to identify which event associations may have been added or removed to handle those.