Linq to Entities, Take(3) from Joined table - linq

Question

I am trying to populate a ViewModel in an MVC app with data from a parent table joined with a child table. The only data I want from the child table is a comma diliminated string from the Nomenclature field of the top three records and put them into a string field in the ViewModel. Here is what I have tried without success:
public IEnumerable<ReqHeaderVM> GetOpenReqs(string siteCode)
{
var openReqs = from h in context.ReqHeaders
join l in context.ReqLineItems on h.ID equals l.ReqID into reqLineItems
select new ReqHeaderVM
{
ReqID = h.ID,
ShopCode = h.ShopCode
Nomenclatures = reqLineItems.Select(x => x.Nomenclature).Take(3) // This doesn't work
};
return (openReqs.ToList());
}
Here is the ViewMdel:
public class ReqHeaderVM
{
[Editable(false)]
public string ReqID { get; set; }
public string ShopCode { get; set; }
public string Nomenclatures {get; set;}
}

Answer 1

Assuming that you have proper relationship (foreign key) between ReqHeaders and ReqLineItems, this should give you what you are looking for...
public IEnumerable<ReqHeaderVM> GetOpenReqs(string siteCode)
{
var openReqs = from h in context.ReqHeaders
select new
{
ReqID = h.ID,
ShopCode = h.ShopCode
Nomenclatures = h.ReqLineItems
.OrderBy(x => x.SomeColumn)
.Select(x => x.Nomenclature)
.Take(3)
};
var openReqsTran = from oreq in openReqs.AsEnumerable()
select new ReqHeaderVM
{
oreq.ReqID,
oreq.ShopCode,
Nomenclatures = string.Join(", ", oreq.Nomenclatures)
};
return (openReqsTran);
}
Note that Nomenclatures is a list of type of Nomenclature.

Answer 2

Yes, the join creates a single Cartesian result set. (think tabular data) what you are attempting to do. To get the results you want you have a few choices.
use lazy loading and iterate over each header querying the line items individually.
pro - simple queries
con - select n+1
query all headers and all line items, but build view model with only the top 3
pro - single query
con - large Cartesian result set query too much data
query all headers and all associated lines individuals
pro - 2 smaller, simpler queries
con - query too many line details.
query all headers and top 3 lines per header in 2 queries
pro - get only the information you require
con - complex query for top 3 lines per header.