使用实体框架和WebApi计算属性

Question

How to correctly handle computed properties in EF model?

My try bellow will fail because of "The entity or complex type 'Invoice' cannot be constructed in a LINQ to Entities query."

Consider method "GetInvoice" as WebApi method with allowed querystring.

static void Main(string[] args)
{
        var invs = GetInvoice();

        invs.FirstOrDefault();

}

public static IQueryable<Invoice> GetInvoice()
{
        var model = new Model();

        IQueryable<Invoice> inv = model.Invocies.Include(t => t.Items).SelectInvoiceData();
        return inv;
}

public static class ExtHelper
{
    public static IQueryable<Invoice> SelectInvoiceData(this IQueryable<Invoice> item)
    {
        return item.Select(c => new Invoice
        {
            LatestItemName = c.Items.FirstOrDefault().Name
        });
    }
}

public class Item
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime CreatedAt { get; set; }
}

public class Invoice
{
    public int Id { get; set; }
    public DateTime CreatedAt { get; set; }

    public string Issuer { get; set; }

    [NotMapped]
    public string LatestItemName { get; set; }

    private ICollection<Item> _items;
    public virtual ICollection<Item> Items
    {
        get { return _items ?? (_items = new Collection<Item>()); }
        set { _items = value; }
    }

}

Answer 1

EntityFramework 6 does not support creating partial entities like this. Either use anonymous type:

return item.Select(c => new
{
    LatestItemName = c.Items.FirstOrDefault().Name
});

Or some DTO class that does not belong to context:

return item.Select(c => new InvoiceDTO
{
    LatestItemName = c.Items.FirstOrDefault().Name
});

However in EF Core it is possible to create entities like in your example.