How to write a Linq that can retrieve all parent table records and total of sub-table record, I mean 'separate' into two parts - linq

Question

Let's say I have two tables, parent table 'P' and sub-table 'S', I usually wrote the Linq like this to get what I want:
var rows = from p in db.P
join s in db.S on p.Id equals s.ParentId into subContent
where (some condition here)
select new{
Id = p.Id,
Title = p.Title
SubContentCount = subContent.Count()
}
It's very simple, but if for some reason I have to pass a parameter into this query when there has one (let's say 'key'), I have to do this (I guess :-):
var rows = from p in db.P
join s in db.S on p.Id equals s.ParentId into subContent
where (some condition here)
select p;
if(!string.IsNullOrEmpty(key)){ // I'm using C#
rows = rows.Where(q => q.Title.Contains(key))
}
And then:
var list = rows.Select(q => new ()
{
Id = q.Id,
Title = q.Title,
subCount = ???.Count()
});
Is that passable to do Linq like this? if so, how?
Thanks for any kind help!

Answer 1

You could create a method that receives a Func<Table, bool>as parameter and use it to filter your dataset:
public static void Main(string[] args)
{
var rows = new List<Table>
{
new Table { Id = 1, Title = "A", SubContent = new [] { "A1" } },
new Table { Id = 2, Title = "B", SubContent = new [] { "B1", "B2" } },
new Table { Id = 3, Title = "C", SubContent = new [] { "C1", "C2", "C3" } },
};
var title = "C";
foreach (var item in Filter(rows, table =>
String.IsNullOrEmpty(title) || table.Title == title))
{
Console.WriteLine(
"Title={0}, SubContent.Length={1}",
item.Title, item.SubContent.Length);
}
}
public static List<Table> Filter(List<Table> original, Func<Table, bool> filter)
{
return original.Where(filter).ToList();
}
public class Table
{
public int Id { get; set; }
public string Title { get; set; }
public string[] SubContent { get; set; }
}

Answer 2

Why not include the filter in the where clause?
where string.IsNullOrEmpty(key) || p.Title.Contains(key)
Quick example in the interactive console:
public class Parent { public int Id {get; set;} public string Title {get; set;} }
public class SubTable { public int Id {get; set;} public int ParentId {get; set;} }
public class Result { public int Id {get; set;} public string Title {get; set;} public int SubContentCount {get; set;} }
var p1 = new Parent() { Id = 1, Title = "Parent_1" };
var p2 = new Parent() { Id = 2, Title = "Parent_2" };
var p3 = new Parent() { Id = 3, Title = "Parent_3" };
var s1_1 = new SubTable() { Id = 11, ParentId = 1 };
var s1_2 = new SubTable() { Id = 12, ParentId = 1 };
var s1_3 = new SubTable() { Id = 13, ParentId = 1 };
var s2_1 = new SubTable() { Id = 21, ParentId = 2 };
var s2_2 = new SubTable() { Id = 22, ParentId = 2 };
var s3_1 = new SubTable() { Id = 31, ParentId = 3 };
var db_P = new List<Parent>() { p1, p2, p3 };
var db_S = new List<SubTable>() { s1_1, s1_2, s1_3, s2_1, s2_2, s3_1 };
public IEnumerable<Result> GetResults(string key = null)
{
var rows = from p in db_P
join s in db_S on p.Id equals s.ParentId into subContent
where string.IsNullOrEmpty(key) || p.Title.Contains(key)
select new Result() {
Id = p.Id,
Title = p.Title,
SubContentCount = subContent.Count()
};
return rows;
}
And example output (formatted onto multiple lines for readability)
> GetResults().ToList()
List<Submission#0.Result>(3) {
Submission#0.Result { Id=1, SubContentCount=3, Title="Parent_1" },
Submission#0.Result { Id=2, SubContentCount=2, Title="Parent_2" },
Submission#0.Result { Id=3, SubContentCount=1, Title="Parent_3" }
}
> GetResults("1").ToList()
List<Submission#0.Result>(1) {
Submission#0.Result { Id=1, SubContentCount=3, Title="Parent_1" }
}
>