EF Core Relationships

In EF Core, you can define relationships between entities to reflect real-world associations. These include one-to-one, one-to-many, and many-to-many relationships.

One-to-Many Relationship

Example: One Category can have many Products

public class Category
{
    public int Id { get; set; }
    public string Name { get; set; }

    public List<Product> Products { get; set; } // Navigation property
}

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }

    public int CategoryId { get; set; } // Foreign key
    public Category Category { get; set; } // Navigation property
}

One-to-One Relationship

Example: Each User has one Profile

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public Profile Profile { get; set; }
}

public class Profile
{
    public int Id { get; set; }
    public string Bio { get; set; }

    public int UserId { get; set; }
    public User User { get; set; }
}

Many-to-Many Relationship

Example: Students and Courses — a student can enroll in many courses and a course can have many students.

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }

    public List<Course> Courses { get; set; }
}

public class Course
{
    public int Id { get; set; }
    public string Title { get; set; }

    public List<Student> Students { get; set; }
}

EF Core 5+ automatically creates a join table for many-to-many relationships.

Fluent API Configuration (Optional)

Use OnModelCreating to configure relationships explicitly:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<User>()
        .HasOne(u => u.Profile)
        .WithOne(p => p.User)
        .HasForeignKey<Profile>(p => p.UserId);
}

Querying with Navigation Properties

Use Include() to load related entities:

var category = context.Categories
    .Include(c => c.Products)
    .FirstOrDefault(c => c.Id == 1);

EF Core makes it easy to model real-world relationships between entities. In the next lesson, we’ll learn how to query these relationships efficiently using LINQ and projection.

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