Beziehungen: Eins-zu-viele und Viele-zu-viele

Beziehungen in EF Core

Eins-zu-viele: Entitäten einrichten

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public ICollection<Post> Posts { get; set; } = new List<Post>();
}

public class Post
{
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public int BlogId { get; set; }        // FK
    public Blog Blog { get; set; } = null!; // nav property
}

Fluent API: Eins-zu-viele

protected override void OnModelCreating(ModelBuilder mb)
{
    mb.Entity<Blog>()
      .HasMany(b => b.Posts)
      .WithOne(p => p.Blog)
      .HasForeignKey(p => p.BlogId)
      .OnDelete(DeleteBehavior.Cascade);
}

Mit Include abfragen (Eager Loading)

// Load blog with all its posts
var blog = await _db.Blogs
    .Include(b => b.Posts)
    .FirstOrDefaultAsync(b => b.Id == 1);

// Load posts with their comments too
var blogs = await _db.Blogs
    .Include(b => b.Posts)
        .ThenInclude(p => p.Comments)
    .ToListAsync();

Viele-zu-viele ohne Join-Entität

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public ICollection<Course> Courses { get; set; } = new List<Course>();
}

public class Course
{
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public ICollection<Student> Students { get; set; } = new List<Student>();
}
// EF creates StudentCourse join table automatically

Viele-zu-viele mit Join-Entität

public class Enrollment
{
    public int StudentId { get; set; }
    public Student Student { get; set; } = null!;
    public int CourseId { get; set; }
    public Course Course { get; set; } = null!;
    public DateTime EnrolledAt { get; set; }
    public decimal? Grade { get; set; }
}

// Fluent API config:
mb.Entity<Enrollment>().HasKey(e => new { e.StudentId, e.CourseId });
mb.Entity<Enrollment>().HasOne(e => e.Student).WithMany(s => s.Enrollments);
mb.Entity<Enrollment>().HasOne(e => e.Course).WithMany(c => c.Enrollments);

Verbundene Daten einfügen

var blog = new Blog { Name = "Dev Notes" };
blog.Posts.Add(new Post { Title = "Hello EF Core" });
blog.Posts.Add(new Post { Title = "Migrations Deep Dive" });

_db.Blogs.Add(blog);
await _db.SaveChangesAsync();
// Inserts Blog + 2 Posts with correct BlogId FK

Nach verbundenen Daten filtern

// Blogs that have at least one published post
var popularBlogs = await _db.Blogs
    .Where(b => b.Posts.Any(p => p.IsPublished))
    .Select(b => new { b.Name, PostCount = b.Posts.Count(p => p.IsPublished) })
    .OrderByDescending(b => b.PostCount)
    .ToListAsync();

Kaskadenlöschen

// Delete a blog and all its posts
var blog = await _db.Blogs.FindAsync(1);
_db.Blogs.Remove(blog!);
await _db.SaveChangesAsync();
// Cascade: all Posts with BlogId=1 are also deleted

Owned-Entitäten für Value Objects

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public Address ShippingAddress { get; set; } = new();
}

public class Address
{
    public string Street { get; set; } = "";
    public string City { get; set; } = "";
    public string Country { get; set; } = "";
}

// Fluent API:
mb.Entity<Customer>().OwnsOne(c => c.ShippingAddress);

Praxisbeispiel: Order mit Bestellpositionen

var order = await _db.Orders
    .Include(o => o.Lines)
        .ThenInclude(l => l.Product)
    .FirstOrDefaultAsync(o => o.Id == orderId);

if (order is null) return Results.NotFound();

var summary = new
{
    OrderId = order.Id,
    Total   = order.Lines.Sum(l => l.Quantity * l.Product.Price),
    Items   = order.Lines.Select(l => new
    {
        l.Product.Name,
        l.Quantity,
        LineTotal = l.Quantity * l.Product.Price
    })
};
return Results.Ok(summary);

Schnelltest

Zusammenfassung: Beziehungen in EF Core