ViewModel Object Convert to Entity Framework Object

问题

Goal: to save ViewModel object by Entity Framework. I have UserViewModel object which has list of UnitViewModel. Then, I have a UserAdapter class which converts UserViewModel into Entity Framework User object (see Convert()below how).

Now, my question is how do I convert this list of UnitViewModel to its corresponding Entity Framework Unit list? - Do I have to get each object from DB Context by calling something like context.Units.Where(u=>myListofUnitIDs.Contains(u.UnitID))?

public class UserViewModel
{
    public Guid? UserID { get; set; }
    public string UserName { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Password { get; set; }
    public DateTime? CreateTime { get; set; }
    public List<UnitViewModel> UserUnits { get; set; }
}


public class UnitViewModel
{
    public Guid UnitID { get; set; }
    public string Name { get; set; }
    public int? SortIndex { get; set; }
    public DateTime CreateTime { get; set; }
    public bool Assigned { get; set; }
}


public class UserAdapter
{
    public static User Convert(UserViewModel userView)
    {
        User user;
        if (userView.UserID.HasValue)
        {
            using (var provider = new CoinsDB.UsersProvider())
            {
                user = provider.GetUser(userView.UserID.Value);
            }
        }
        else
        {
            user = new User();
        }

        user.FirstName = userView.FirstName;
        user.LastName = user.LastName;
        user.Password = StringHelper.GetSHA1(userView.Password);
        user.UserName = user.UserName;
        user.CreateTime = DateTime.Now;

        // Problem here :)
        // user.Units = userView.UserUnits;

        return user;
    }
}

UPDATE: The main concern here is that I have to retrieve each Unit from database to match (or map) it with ViewModel.Unit objects, right? Can I avoid it?

回答1:

For your information, this operation is called as Mapping mainly. So, you want to map your view model object to the entity object.

For this, you can either use already existed 3rd party library as AutoMapper. It will map properties by reflection which have same name. Also you can add your custom logic with After method. But, this approach has some advantages and disadvantages. Being aware of these disadvantages could help you to decide whether you must use this API or not. So, I suggest you to read some articles about advantages and disadvantages of AutoMapper especially for converting entities to other models. One of such disadvantages is that it can be problem to change the name of one property in the view model in the future, and AutoMapper will not handle this anymore and you won't get any warning about this.

foreach(var item in userView.UserUnits)
{
     // get the mapped instance of UnitViewModel as Unit
     var userUnit = Mapper.Map<UnitViewModel, UserUnit>(item);
     user.Units.Add(userUnit);
}

So, I recommend to write your custom mappers. For example, I have created a custom library for this and it maps objects lik this:

 user.Units = userView.UserUnits
       .Select(userUnitViewModel => userUnitViewModel.MapTo<UserUnit>())
       .ToList();

And I am implementing these mapping functions as:

 public class UserUnitMapper:
        IMapToNew<UnitViewModel, UserUnit>
    {
        public UnitViewModel Map(UserUnit source)
        {
            return new UnitViewModel
            {
                Name = source.Name,
                ...
            };
        }
    }

And then in runtime, I am detecting the types of the objects which will be used during mapping, and then call the Map method. In this way, your mappers will be seperated from your action methods. But, if you want it urgently, of course you can use this:

foreach(var item in userView.UserUnits)
{
     // get the mapped instance of UnitViewModel as Unit
     var userUnit= new UserUnit()
           {
               Name = item.Name,
               ...
           };

     user.Units.Add(userUnit);
}

来源：https://stackoverflow.com/questions/34218257/viewmodel-object-convert-to-entity-framework-object