Relationships in EF Core

EF Core models relationships—one-to-many, many-to-many, one-to-one—with navigation properties and foreign keys, similar to Django ForeignKey and ManyToManyField.

One-to-many

public class Order {
    public int Id { get; set; }
    public List<OrderLine> Lines { get; set; } = new();
}
public class OrderLine {
    public int Id { get; set; }
    public int OrderId { get; set; }
    public Order Order { get; set; } = null!;
}

Eager loading

var order = await _db.Orders
    .Include(o => o.Lines)
    .FirstAsync(o => o.Id == id);

Fluent API

Configure delete behaviors, composite keys, and indexes in OnModelCreating when conventions aren't enough.

Important interview questions and answers

1. Q: Include vs projection?
   A: Include loads full related entities; Select projects only needed fields—often faster for APIs.
2. Q: Cascade delete?
   A: Deleting parent may delete children—configure intentionally to avoid accidental data loss.

Self-check

1. What property holds the FK on the many side?
2. How do you load Order with Lines in one query?

Tip: N+1 queries happen when you loop entities and lazy-load navigation properties—use Include or project with Select in one query.