LINQ JOIN ON TOP 1 ROW - sql

Question

I would like to do a LINQ join including only one row. In SQL, that's what I want to do :
JOIN Person ON ( SELECT TOP (1) top1Person.ID FROM Person AS top1Person WHERE top1Person.CompanyID = Company.ID ) = Person.ID
But I am not able to do it in LINQ, I tried this :
join pers in ctx.Persons on cmp.Persons.First().ID equals pers.ID
but the Fisrt() method is not allowed here...
Someone could help me ? Thanks

Answer 1

There are many ways to accomplish that. One way would be to use First() or FirstOrDefault() on the result of query. Try this:
var results = (from id in ids
join p in persons on id equals p.ID
where p.CompanyID == companyID
select p).FirstOrDefault();
You can do OrderBy(x => x.ID).FirstOrDefault() first if you want the lowest ID first, or OrderByDesc(x => x.ID).FirstOrDefault() if you want the highest first.
EDIT: Upon further inspection of your sample code, it looks like this may be more of what you're looking for.
var results = (from company in companies
join p in persons on persons.First(x => x.CompanyID == company.ID).ID equals p.ID
select p);
You should be able to call First() in that spot, but it looks like your LINQ query doesn't quite match up with your SQL. Should you be doing ctx.Persons.First() intstead of cmp.Persons.First()? If the above solution doesn't work for you, then we need more information.
FINAL EDIT: One last full solution for a slightly different interpretation. This code is tested and works.
using System;
using System.Collections.Generic;
using System.Linq;
namespace com.test
{
public class Program
{
public static void Main(string[] args)
{
List<int> ids = new List<int>();
List<Company> companies = new List<Company>();
List<Person> persons = new List<Person>();
persons.Add(new Person(1, 1, "John Smith"));
persons.Add(new Person(2, 1, "Adam Jones"));
persons.Add(new Person(3, 1, "Alex Rabbit"));
persons.Add(new Person(4, 2, "Jessica Thurman"));
persons.Add(new Person(5, 2, "Sam Riot"));
persons.Add(new Person(6, 2, "Donald Lewis"));
persons.Add(new Person(7, 3, "Lindsay Bonaparte"));
persons.Add(new Person(8, 3, "Desmond Tutu"));
persons.Add(new Person(9, 3, "Kevin Gargoyle"));
persons.Add(new Person(10, 4, "Emily Francis"));
persons.Add(new Person(11, 4, "Caitlin Elizabeth"));
persons.Add(new Person(12, 4, "Harry Finstein"));
persons.Add(new Person(13, 4, "Carla Loper"));
companies.Add(new Company(1, "McDonalds", persons.Where(x => x.CompanyID == 1).ToList()));
companies.Add(new Company(2, "Burger King", persons.Where(x => x.CompanyID == 2).ToList()));
companies.Add(new Company(3, "Wendy's", persons.Where(x => x.CompanyID == 3).ToList()));
companies.Add(new Company(4, "Arby's", persons.Where(x => x.CompanyID == 4).ToList()));
var results = (from cmp in companies
join p in persons on cmp.Persons.First().ID equals p.ID
select p);
foreach (var p in results)
{
Console.WriteLine("Person: " + p.Name);
}
Console.ReadKey();
}
public class Person
{
public int ID { get; set; }
public int CompanyID { get; set; }
public string Name { get; set; }
public Person (int id, int companyID, string name)
{
ID = id;
CompanyID = companyID;
Name = name;
}
}
public class Company
{
public int ID { get; set; }
public string Name { get; set; }
public List<Person> Persons { get; set; }
public Company(int id, string name, List<Person> persons)
{
ID = id;
Name = name;
Persons = persons;
}
}
}
}