.Net如何使用数据库中不存在的字段加载ViewModel？

Question

How can I query the DB to return a list of items that have fields in addition to what the entity models describe>

Say I have entity models called Owners and Pets:

public class Owner
{
    public int OwnerID{ get; set; }

    public virtual IEnumerable<Pets> Pets{ get; set; }
}

public class Pet
{
    public int PetID{ get; set; }
    public string PetName{ get; set; }

    public int OwnerID{ get; set; }
    public Owner Owner{ get; set; }
}

If I got a list of owners like this:

dbContext.Set<Owner>().ToList()

Each object in that list would be an Owner with OwnerID and a list of their pets.

But, what if I wanted to include an extra field for each of those Owners, like "HasPets", and have a list of viewmodels that accepts that.

public class OwnerViewModel
{
    public int OwnerID{ get; set; }
    public bool HasPets{ get; set; }
}

List<OwnerViewModel> OwnerViewModels = ....(get list with added field)

I have no idea how to query the DB to include a new calculated field...

(I realize I could simply do a count on Owner.Pets, but I just wanted to keep the example simple)

Answer 1

@Kamyar is pretty much right, the only thing I would do differently is use .Any() as opposed to .Count() > 0

context.Owners
            .Select(o => new OwnerViewModel
                                {
                                    OwnerID = o.OwnerID,
                                    HasPets = o.Pets.Any().
                                    ...
                                    ...
                                });

Answer 2

Something like

context.Owners.Select(o => new OwnerViewModel() 
                           {Owner = o, HasPets = o.Pets.Any()});  

And define your viewmodel like:

public class OwnerViewModel
{
    public Owner Owner{ get; set; }
    public bool HasPets{ get; set; }
}

Or you can load all the pets client-side, and check them there (of course not recommended for the overload):

public class OwnerViewModel
{
    public Owner Owner{ get; set; }
    public bool HasPets{ get { return this.Owner.Pets.Any(); } }
}  
var ownervms = new List<OwnerViewModel>();
foreach (var owner in context.Owners)
{
    ownervms.Add(new OwnerViewModel() {Owner= owner});
}