Question

I’m using LINQ to Entities.

I have a table called Student; it has ID and Name as it’s columns. ID is a primary key.

I’d like to be able select the name of the Student and get the amount of Students with the same Name.

So for example I’d have this as my table data.

ID  Name  
1   Bob
2   Will
3   Bob

After performing the query I’d return a List of Student objects looking like this.

Name    Quantity
Bob     2
Will    1

I guess it is kind of similar to how the Tags page of stackoverflow works; It has the name and the quantity.

Anyways, I created a partial class called Student.cs in which I added a Quantity property like this.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace MySite.Models
{
    public partial class Student
    {
        private int _quantity;

        public int Quantity
        {
            get { return _quantity; }
            set { _quantity = value; }
        }
    }
}

I came up with this but I’m getting an error..

    public IQueryable<Student> FindStudentsDistinctWithQuantity()
    {
        /*SELECT Name, COUNT(Name) AS Quantity
        FROM Student
        GROUP BY Name*/

        var students= (from s in db.Students
                    group s by s.Name into g
                    select new {Name = g.Key, Quantity = g.Count()});            

        return students;
    }

The error i’m getting says something like Can’t convert from type Anonymous to the Student list. Does it have something to do with it not recognizing the quantity field I added in the partial class?

Thanks!

Answer 1

Change your Student type to look like this:

public partial class Student
{
    public Int32 Quantity { get; set; }
    public String Name { get; set; }
}

And your query to look like this:

var students = from s in db.Students
               group s by s.Name into g
               select new Student { 
                   Name = g.Key, 
                   Quantity = g.Count() };

Your method returns an IQueryable<Student> but you are currently returning an IQueryable<T> of a projected anonymous type.

You need to refactor your Student type to have a Name property of type String and then project new instances of your Student type from the expression so that the return type of your expression will match the return type of your method.