Koan Relationships

Core Principle

Relationships use foreign keys + navigation helpers. Batch load to prevent N+1 queries. No ORM mapping configuration needed.

Relationship Patterns

One-to-Many

public class User : Entity<User>
{
    public string Name { get; set; } = "";

    public Task<List<Todo>> GetTodos(CancellationToken ct = default) =>
        Todo.Query(t => t.UserId == Id, ct);
}

public class Todo : Entity<Todo>
{
    public string UserId { get; set; } = "";

    public Task<User?> GetUser(CancellationToken ct = default) =>
        User.Get(UserId, ct);
}

Preventing N+1 Queries

// ❌ WRONG: N+1 query problem
foreach (var todo in todos)
{
    var user = await todo.GetUser(); // N queries!
}

// ✅ CORRECT: Batch load
var userIds = todos.Select(t => t.UserId).Distinct().ToArray();
var users = await User.Get(userIds);
var userDict = users.Where(u => u != null).ToDictionary(u => u!.Id);

foreach (var todo in todos)
{
    var user = userDict[todo.UserId];
}

Optional Relationships

public class Todo : Entity<Todo>
{
    public string? CategoryId { get; set; }

    public Task<Category?> GetCategory(CancellationToken ct = default) =>
        string.IsNullOrEmpty(CategoryId) ? Task.FromResult<Category?>(null)
            : Category.Get(CategoryId, ct);
}

Hierarchical (Parent-Child)

public class TodoItem : Entity<TodoItem>
{
    public string TodoId { get; set; } = "";
    public int SortOrder { get; set; }

    public Task<Todo?> GetParentTodo(CancellationToken ct = default) =>
        Todo.Get(TodoId, ct);
}

public class Todo : Entity<Todo>
{
    public Task<List<TodoItem>> GetItems(CancellationToken ct = default) =>
        TodoItem.Query(i => i.TodoId == Id, ct)
            .ContinueWith(t => t.Result.OrderBy(i => i.SortOrder).ToList());
}

When This Skill Applies

• ✅ Complex data relationships
• ✅ Navigation patterns
• ✅ Performance optimization (N+1)
• ✅ Hierarchical data
• ✅ Optional relationships

Reference Documentation

• Example Code: .claude/skills/entity-first/examples/entity-relationships.cs
• Sample: samples/S1.Web/README.md (Relationship demo)