I get a ModelValidationException (at the bottom) when working with "EF-Code First". It wants me to define a Key but I'm not sure what exactly it means...

```
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.
> 
> 
>