How to configure a one-to-one relationship in EF Core?

Short answer: Example using Fluent API: modelBuilder.Entity<User>() .HasOne(u => u.Profile) .WithOne(p => p.User) .HasForeignKey<Profile>(p => p.UserId); Or with data annotations: public class Profile

Example code

{ [Key, ForeignKey("User")] public int UserId { get; set; }
public User User { get; set; }
} Requires a unique foreign key. EF assumes optional by default unless marked as required.

Real-world example (ShopNest)

ShopNest’s order service uses EF Core to save an Order and its line items in one SaveChangesAsync() call inside a transaction.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.