LINQ Query for populating collection inside another collection

Question

I am looking forward to get a linq query for populating list of teachers and their respective divisons. Here I have 2 classes Teacher and Division which are related by DivisionGroupID - GroupID

public class Teacher
{
    public int ID { get; set; }
    public string Name { get; set; }
    public List<Division> lstDivison {get;set;}
    public int DivisionGroupID { get; set; }
    
}

public class Division
{
    public int GroupID { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }  
}

In main method List of both Teacher and Division will be populated

      static void Main(string[] args)
    {
        Teacher obj = new Teacher { ID = 1, DivisionGroupID = 11, Name = "abcd" };
        Teacher obj1 = new Teacher { ID = 2, DivisionGroupID = 12, Name = "efgh" };
        List<Teacher> objList = new List<Teacher>();
        objList.Add(obj);
        objList.Add(obj1);
        Division dv = new Division { GroupID = 11 ,Name="Division1",Description="first" };
        Division dv1 = new Division { GroupID = 11, Name = "Division2", Description = "second" };
        Division dv2 = new Division { GroupID = 11, Name = "Division3", Description = "third" };
        Division dv3 = new Division { GroupID = 12, Name = "Division4", Description = "fourth" };
        Division dv4 = new Division { GroupID = 12, Name = "Division5", Description = "fifth" };
        Division dv5 = new Division { GroupID = 12, Name = "Division6", Description = "sixth" };
        List<Division> lstDiv = new List<Division>();
        lstDiv.Add(dv);
        lstDiv.Add(dv1);
        lstDiv.Add(dv2);
        lstDiv.Add(dv3);
        lstDiv.Add(dv4);
        lstDiv.Add(dv5);

}

The requirement here is to get the list of teachers and populate the sublist of divisions each teachers holding. I got the solution based on 2 approaches.

Using sub query approach :

var upd = from teacher in objList
          select new Teacher
          {
              ID = teacher.ID,
              Name = teacher.Name,
              lstDivison = (from div in lstDiv
                            where div.GroupID == teacher.DivisionGroupID
                            select new Division
                            {
                                Name = div.Name,
                                Description = div.Description
                            }).ToList()
          };

Using Foeach loop through Teacher collection(objList) and updating the lstDivision

  objList.ForEach(x => x.lstDivison = lstDiv
    .Where(y => y.GroupID == x.DivisionGroupID)
    .Select(p => new Division { Name = p.Name, Description = p.Description })
    .ToList());

Both of these approaches will give me the result. But i am looking forward a better approach in as part of my project requirement which has to improve the query performance. Could you please suggest which is the best approach to handle this situation?

_"Using Foeach loop"_ Fyi, you are using [`List.ForEach`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.foreach?view=net-5.0) which is not a [`foreach`](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/iteration-statements#the-foreach-statement)-loop. It's also using a loop internally but it's a method. — Tim Schmelter, May 31 '21 at 14:35

score 0 · Answer 1 · answered Jun 01 '21 at 15:09

use yours teacher object to populate list of divisions under it. as my understanding that how it was designed class structure.

   //populate property in object
    objList.ForEach(x => {
        x.lstDivison = lstDiv.Where(w=> w.GroupID == x.DivisionGroupID).ToList();   
    });

    objList.Dump();

LINQ Query for populating collection inside another collection

1 Answers1