Model Restraints

One-to-many relationships

UserType belongs to many Users <-> Users have one UserType

One way navigation property with required

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);

One way navigation property with optional (foreign key must be Nullable type)

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);

Two way navigation property with (required/optional change the foreign key property as needed)

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;}
}

Required

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

Optional

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