如何基于 EF Core（无密钥实体）中的查询创建对象？

Question

I am trying to map the results of a complex SQL query to a keyless entity in EF Core. (I basically need to create a view client-side due to having to deal with a preexisting database.)

However, when trying to retrieve my data, I get an InvalidOperationException "Sequence contains no elements". I can map base tables in the DB to normal entities, so I know the connection string is correct. A simplified, but complete example:

using System;
using System.Linq;
using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

class Program
{
    static void Main(string[] args)
    {
        using (var db = new QueryDbContext())
        {
            List<Model> result = db.Orders.ToList();  // <-- exception occurs here.
            Console.WriteLine($"Found: {result.Count}");
        }
    }
}

public class Model
{
    public string OrderId { get; }
}

public class QueryDbContext : DbContext
{
    public QueryDbContext()
        : base()
    {}

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            _ = optionsBuilder
                .UseSqlServer("myconnectionstring");
        }
    }

    public DbSet<Model> Orders { get; set; }
    private const string _OrdersSql =
        "select top 5 orderid from tbl_orders order by transacttime desc";

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder
            .Entity<Model>(m =>
            {
                m.HasNoKey();
                m.ToQuery(() => Set<Model>().FromSqlRaw(_OrdersSql));
            });
    }
}

Set<>() is the current version (since EFCore 3.0) of Query<>() as described in the answer to FromSql with Non-Existing Entity .

Answer 1

The problem was that the model properties need both setters and getters, even though the model is read-only. So the model should be

public class Model
{
    public string OrderId { get; set; }
}