Linq strategy for complex set - sql

Question

Am developing a ViewModel/PresentationModel which is getting complex.
I want the Linq query to return an IQueryable<UserPresentationModel>
Using EntityFramework against MSSQL
Is it possible to do any sort of iteration over the set before returning it to the presentation layer ie
List<UserPresentationModel> list = new List<UserPresentationModel>();
foreach (var person in listOfPeople)
{
UserPresentationModel u = new UserPresentationModel();
int userUIStatus = GetColourStateOfPerson(person);
u.FirstName = person.FirstName;
u.UserUIStatus = userUIStatus;
list.Add(u);
}
return list
This feels like it would always be N+1, and I'd never get the advantages of deferred execution, composing of queries..
Or (and I think am answering my own question) do I need to think in a SQL set based manner.

Answer 1

First, we can convert your code to LINQ.
IEnumerable<UserPresentationModel> models =
from person in listOfPeople
select new UserPresentationModel
{
FirstName = person.FirstName,
UserUIStatus = GetColourStateOfPerson(person)
}
return models.ToList();
Now, if GetColourStateOfPerson is making a DB round-trip, you definitely want to pull that out.
IDictionary<int, int> colourStatesByPersonId = GetColourStatesOfPeople(listOfPeople);
IEnumerable<UserPresentationModel> models =
from person in listOfPeople
select new UserPresentationModel
{
FirstName = person.FirstName,
UserUIStatus = colourStatesByPersonId[person.PersonId]
}
return models.ToList();
You could probably manage to create a single LINQ query that grabs just the first names and colour states of the people you want in a single query, but you haven't provided enough information about your data context for me to help you with that.
I would personally avoid passing around an IQueryable, which could continue making database trips any time somebody touches it. Let your data layer get out all the data you're likely to need, compose it into a list, and return that.

Answer 2

use IEnumerable<T>.Aggregate() instead of looping.
return listOfPeople.Aggregate(new List<UserPresentationModel>(), person => {
return new UserPresentationModel {
FirstName = person.FirstName,
UserUIStatus = GetColourStateOfPerson(person)
};
}).AsQueryable();

Answer 3

return listOfPeople.AsEnumerable().Select(p =>
new UserPresentationModel
{
FirstName = p.FirstName,
UserUIStatus = GetColourStateOfPerson(p)
}).AsQueryable();
I'm assuming that listOfPeople is an IQueryable that will eventually execute against your database. If that is the case then AsEnumerable() is important because SQL Server won't know what to do with GetColourStateOfPerson(). AsEnumerable() will force the IQueryable's expression tree to execute, pull the resulting rows out of your database and then apply Select() transformation in code as oppose to in SQL Server.
If you can implement GetColourStateOfPerson() as a stored proc or database function then you can omit AsEnumerable() and AsQueryable() and allow execution to delay even longer.