https://twitter.com/ravindUwU/status/1625038749406232578

1.cs

// Context:
// - Invoked within `OnModelCreating(ModelBuilder modelBuilder)`.
// - `entity` is an `IMutableEntityType`, obtained by iterating `modelBuilder.Model.GetEntityTypes()`.
// - `builder` is an `EntityTypeBuilder`, obtained by calling `modelBuilder.Entity(string name)`.

// Find [Key] properties of the entity.
var keys = entity.GetProperties()
	.Where((p) => p.PropertyInfo?.GetCustomAttribute<KeyAttribute>() is not null)
	.ToList();

// If the entity has a composite key, register it with `EntityTypeBuilder.HasKey`.
if (keys.Count > 1)
{
	// Group [Key]s by their [Column] order.
	var columnOrders = keys
		.Where((p) => p.PropertyInfo is not null)
		.GroupBy((p) => p.PropertyInfo!.GetCustomAttribute<ColumnAttribute>()?.Order
			?? throw new Exception("[Column(Order = ?)] must be specified on all [Key]s.")
		);

	// Ensure that [Key]s are of distinct [Column] orders (i.e., groups can't have > 1 column),
	var badColumnOrders = columnOrders.Where((g) => g.Count() > 1);
	if (badColumnOrders.Any())
	{
		var m = String.Join(", ", badColumnOrders.Select((g) => $"Order {g.Key}: [{String.Join(", ", g.Select((p) => p.Name))}]"));
		throw new Exception($"[Key]s must be ordered distinctly. {m}");
	}

	// Define ordered composite key for the entity.
	var key = columnOrders
		.OrderBy((g) => g.Key)
		.Select((g) => g.Single().Name)
		.ToArray();

	builder.HasKey(key);
}

2.cs

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

public class MyTable
{
	[Key]
	[Column(Order = 0)]
	public string Key1 { get; set; } = default!;

	[Key]
	[Column(Order = 1)]
	public string Key2 { get; set; } = default!;

	// ...
}