LINQ how to order on a referenced List - linq

Question

I have an object as defined like this
public class Person {
public Person() {
this.NewsItems = new List<NewsItem>();
}
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public int Age {get;set;}
public virtual IList<NewsItem> NewsItems { get; set; }
}
The NewsItem object has a property called DisplayOrder and it is of type int. I then have LINQ as following:
return _repo.GetAll<Person>().Where(p => p.age >60).ToList();
My objective is have a sorted List<Person> based on DisplayOrder.
I am going to use this List<Person> in controller. In this controller I do have access to Person.NewsItems. I need to have List<Person> to be sorted in order or DisplayOrder which is inside the referenced list NewsItems.
Basically I have 1:m relationship between Person and NewsItem.

Answer 1

You can implement a custom comparer that you can use with .OrderBy. This could be against the NewsItems property. So you can do something like:
var sortedPersons = persons
.Where(p => p.Age > 60)
.OrderBy(x => x.NewsItems, new NewsItemsComparer());
Where your NewsItemComparer could take the form of:
public class NewsItemsComparer : IComparer<IList<NewsItem>>
{
public int Compare(IList<NewsItem> x, IList<NewsItem> y)
{
if (<logic where xDisplayOrders takes precedence over yDisplay Orders>)
{
return 1;
}
if (<logic where yDisplayOrders takes precedence over xDisplay Orders>)
{
return -1;
}
else
return 0;
}
}
Here's a sample implementation in which Persons are sorted according to which NewsItems has the lowest DisplayOrder number: https://dotnetfiddle.net/MTtwHU