asp.net – 在使用“EF-Code First”时如何定义Keys？

public class Unit
{
    Guid id;
    String public_id;
    String name;        
    bool deleted;
}

public class MyDataContext : DbContext
{
    public DbSet<Unit> Units { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Unit>().ToTable("sometable");
    }
}

[TestFixture]
public class DataTests
{
    [Test]
    public void Test()
    {
        MyDataContext database = new MyDataContext();
        var o = database.Units;


        Console.WriteLine(o.Count()); // This line throws!
        Assert.IsTrue(true);
    }
}

System.Data.Entity.ModelConfiguration.ModelValidationException : One or more validation errors were detected during model generation:

System.Data.Edm.EdmEntityType: : EntityType ‘Unit’ has no key defined. Define the key for this EntityType.

System.Data.Edm.EdmEntitySet: EntityType: The EntitySet Units is based on type Unit that has no keys defined.