When we have a one-to-one relationship, we can say that a record in one table will only be able to have a relationship with exactly one record in another table. 

For example, if we have a User class and a UserAvatar class, and a user has one and only one avatar, then we can represent this using the Fluent API, as follows:

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
            .HasOne(u => u.UserAvatar)
            .WithOne(a => a.User)
            .HasForeignKey<UserAvatar>(u => u.UserForeignKey);
    }

In the preceding code snippet, we have a one-to-one relationship between the user and their avatar. This is represented by the code in bold, that is, .HasOne(u => u.UserAvatar).WithOne(a => a.User). It applies both ways.