一对多的关系

UserType 属于许多用户< - >用户有一个 UserType

单向导航属性需要

public class UserType
{
    public int UserTypeId {get; set;}
}
public class User
{
    public int UserId {get; set;}
    public int UserTypeId {get; set;}
    public virtual UserType UserType {get; set;}
}

Entity<User>().HasRequired(u => u.UserType).WithMany().HasForeignKey(u => u.UserTypeId);

单向导航属性可选（外键必须是 Nullable 类型）

public class UserType
{
    public int UserTypeId {get; set;}
}
public class User
{
    public int UserId {get; set;}
    public int? UserTypeId {get; set;}
    public virtual UserType UserType {get; set;}
}

Entity<User>().HasOptional(u => u.UserType).WithMany().HasForeignKey(u => u.UserTypeId);

双向导航属性（必要/可选根据需要更改外键属性）

public class UserType
{
    public int UserTypeId {get; set;}
    public virtual ICollection<User> Users {get; set;}
}
public class User
{
    public int UserId {get; set;}
    public int UserTypeId {get; set;}
    public virtual UserType UserType {get; set;}
}

需要

Entity<User>().HasRequired(u => u.UserType).WithMany(ut => ut.Users).HasForeignKey(u => u.UserTypeId);

可选的

Entity<User>().HasOptional(u => u.UserType).WithMany(ut => ut.Users).HasForeignKey(u => u.UserTypeId);