Using a custom method in LINQ to Entities - asp.net

Question

I have a LINQ expression:
var users = db.Relationships.Where(i => i.RelationshipId== relId)
.Select(s => s.User).Distinct().Select(s => new UserViewModel() {
Username = s.Username,
LastActiveDateTime = s.LastActive, // DateTime, I want it to be a string filtered through my custom GetFriendlyDate method
}).ToList();
// convert all DateTimes - yuck!
foreach (var userViewModel in users) {
userViewModel.LastActive = DateFriendly.GetFriendlyDate(userViewModel.LastActiveDateTime);
}
This solution is working, but it feels wrong to
have to iterate over all users after getting them from the db, just to reformat a property on every single one
have a DateTime property on my ViewModel just so that it can later be converted to a string and never touched again
Is there any way I can use the GetFriendlyDate method directly within the query?

Answer 1

Possible solutions, worth to mention:
Have a getter property of your ViewModel, which would return transformed string, something like:
public string LastActive
{
get
{
return DateFriendly.GetFriendlyDate(LastActiveDateTime);
}
}
Though it not solves your problem with existing LastActiveDateTime column, transformation will be applied only at moment of usage (in your view, most likely - anyways if you will try use it somewhere latter in query, it will not work for reason you already know), so no need to iterate manually.
Create View, which will transform data on server side; so your data will already be returned in format you need, if you're using DBFirst, probably, it's easiest and fastest solution;
Finally, you can use ToList() twice, once before selecting new ViewModel() (or call AsEnumerable(), or find other way to materialize query). It will fetch data from database and will allow you perform any C#-side functions you want directly in query after ToList(). But, as mentioned before - it's about getting all data, which matched criteria up to ToList() - in most cases it's not appropriate solution.
And here is some additional readings:
How can I call local method in Linq to Entities query?

Answer 2

I tested it in LINQpad and it works.
It still kinda iterates over users (with LINQ) but you don't have to add DateTime property to your viewmodel class. Also you could convert collection of Users to collection of UserViewModel objects with Automapper. It would still iterate over users of course but you wouldn't see it.
Had to create some setup code of course because I don't have your database.
void Main()
{
var db = new List<User> {
new User { LastActive = DateTime.Now, Username = "Adam", Lastname = "Nowak" },
new User { LastActive = DateTime.Now.AddYears(1), Username = "Eve", Lastname = "Kowalska"}
};
// select only properties that you need from database
var users = db
.Select(user => new { Username = user.Username, LastActive = user.LastActive})
.Distinct()
.ToList();
var usersVM = from u in users
select new UserViewModel { Username = u.Username, LastActiveDateTime = DateFriendly.GetFriendlyDate(u.LastActive)};
usersVM.Dump();
}
class User
{
public DateTime LastActive;
public string Username;
public string Lastname;
};
class UserViewModel
{
public string Username;
public string LastActiveDateTime;
}
static class DateFriendly
{
public static string GetFriendlyDate(DateTime date)
{
return "friendly date " + date.Year;
}
}
And this outputs
Username LastActiveDateTime
Adam friendly date 2013
Eve friendly date 2014

Answer 3

There is no direct Concert.ToDate method available for LINQ. But you can try using the DateAdd method from the SqlFunctions class:
var users = db.Relationships.Where(i => i.RelationshipId== relId)
.Select(s => new
{
s.User.Username,
LastActive=SqlFunctions.DateAdd("d",0, s.LastActive)
})
.ToList().Select(s => new UserViewModel()
{
Username = s.Username,
LastActiveDateTime = s.LastActive
});

Answer 4

Wouldn't the following work?
var users = db.Relationships.Where(i => i.RelationshipId== relId)
.Select(s => s.User).Distinct().Select(s => new UserViewModel() {
Username = s.Username,
LastActiveDateTime = DateFriendly.GetFriendlyDate(s.LastActive)
}).ToList();