asp.net mvc - Should I have each ViewModel for each entity in my ASP MVC project? -

i have read many many solution can't understand or when use view model? example, when have register form user register, want hava field confirm password, don't think should add user entity. have viewmodel:

public class registerviewmodel {     public user user { get; set; }     public ienumerable<selectlistitem> city { get; set; }     public ienumerable<selectlistitem> ward { get; set; }     [required(errormessage = "bạn chưa nhập lại mật khẩu.")]     [stringlength(100, errormessage = "mật khẩu phải có ít nhất {2} ký tự.", minimumlength = 6)]     [datatype(datatype.password)]     [system.web.mvc.compare("user.password", errormessage = "mật khẩu không khớp.")]     public string confimpass { get; set; } } 

so after read link how implement "confirm password" in asp.net mvc 3? . don't know why should replace password field in user entity. i'm using unobstrusive client validation work if use model view. in view, must use m=> m.user.username not m=>m.username, etc... because of this, validation such compare password, or remote validation not work name in view m=>m.user.username. wrong structure or model view in thinking?

there no single rule , need stay pragmatic, having said viewmodel , model (or domain model) 2 different things. no don't pollute entities placing properties don't belong them. idea ui should interchangeable , domain should not in way depend on it. dependencies should inverted. maybe tomorrow you'd switch (or extend) ui layer wpf (for example) ? current viewmodels (with attributes) wouldn't make sense. in case, yes should creating view model , keep relevant view in them, after map/pass values domain model. hope i'm making sense, let me know if need clarifications.

in case i'd create flattened registerviewmodel include information needed register user, example:

public class registerviewmodel {     [required]     public string displayname { get; set; }     [required]     public string firstname { get; set; }      // etc ...      public ienumerable<selectlistitem> city { get; set; }     public ienumerable<selectlistitem> ward { get; set; }     [required(errormessage = "bạn chưa nhập lại mật khẩu.")]     [stringlength(100, errormessage = "mật khẩu phải có ít nhất {2} ký tự.", minimumlength = 6)]     [datatype(datatype.password)]     [system.web.mvc.compare("user.password", errormessage = "mật khẩu không khớp.")]     public string confimpass { get; set; } }