Using the ALL operator in linq to filter child items of EntitySet

Question

I have a two objects as follows:

public class Item
{
    public int ItemId {get;set;}
    public string ItemName {get;set;}
    public List ItemTa         


        

Answer 1

I think you need to do something like this:

var query = itemList.OrderByDescending(p => p.DateCreated).ToList();

var results = query.Where(i => i.ItemTags
   .All(it => tagsList.Contains(it.TagName.ToLower())));

Then results should then be a list of matching items.

PS. Your code shows you fetching itemList as a List from your repository and then sorting by date created. This means the sorting isn't being done in the database. Once you turn something into a List you give up the benefits of deferred execution as you will bring back the entire collection into memory.

EDIT: Here's the test code to prove it works in Linq to Objects:

public class Item
{
    public int ItemId { get; set; }
    public string ItemName { get; set; }
    public List<Tag> ItemTags { get; set; }
    public DateTime DateCreated { get; set; }
}

public class Tag
{
    public int TagId { get; set; }
    public string TagName { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        RunTags();
    }

    private static void RunTags()
    {
        Item i1 = new Item()
        {
            ItemId = 1,
            ItemName = "Item1",
            ItemTags = new List<Tag>() { new Tag { TagId = 1, TagName = "2008" }, new Tag { TagId = 2, TagName = "Donkey" } }
        };

        Item i2 = new Item()
        {
            ItemId = 2,
            ItemName = "Item2",
            ItemTags = new List<Tag>() { new Tag { TagId = 4, TagName = "Cat" }, new Tag { TagId = 2, TagName = "Donkey" }, new Tag { TagId = 3, TagName = "Seattle" } }
        };

        Item i3 = new Item()
        {
            ItemId = 3,
            ItemName = "Item3",
            ItemTags = new List<Tag>() { new Tag { TagId = 523, TagName = "Manchester united" }, new Tag { TagId = 10, TagName = "European Cup" }, new Tag { TagId = 1, TagName = "2008" } }
        };

        Item i4 = new Item()
        {
            ItemId = 4,
            ItemName = "Item4",
            ItemTags = new List<Tag>() { new Tag { TagId = 05, TagName = "Banana" }, new Tag { TagId = 140, TagName = "Foo" }, new Tag { TagId = 4, TagName = "Cat" } }
        };

        Item i5 = new Item()
        {
            ItemId = 5,
            ItemName = "Item5",
            ItemTags = new List<Tag>() { new Tag { TagId = 05, TagName = "Banana" }, new Tag { TagId = 140, TagName = "Foo" } }
        };

        List<Item> itemList = new List<Item>() { i1, i2, i3, i4, i5 };

        string tags = "Manchester United,European Cup,2008";
        List<string> tagsList = tags.Trim().ToLower()
            .Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries)
            .Distinct(StringComparer.CurrentCultureIgnoreCase)
            .ToList();

        var query = itemList
            .OrderByDescending(p => p.DateCreated).ToList();


        var results = query.Where(i => i.ItemTags.All(it => tagsList.Contains(it.TagName.ToLower())));

        foreach (var item in results)
        {
            Console.WriteLine(item.ItemName); // Should return "Item3"
        }

        Console.ReadLine();
    }

If you want to match any of the tags in the Item's ItemTag list then just change All to Any i.e.

var results = query.Where(i => i.ItemTags.Any(it => tagsList.Contains(it.TagName.ToLower())));

Answer 2

That because you're puting the tags from the items to lowercase, but not the searched tags.

With this modification it should work:

List<string> tagsList = tags
    .Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries)
    .Select(s => s.ToLower())
    .Distinct()
    .ToList();

---

EDIT: OK, I see what the problem is: you're doing it backwards. You're searching for items that have only the tags that you're looking for.

Try that instead:

query = 
    (from item in query
     let itemTags = p.ItemTags.Select(it => it.TagName.ToLower())
     where tags.All(t => itemTags.Contains(t))
     select item).ToList();

---

UPDATE: here's a version with the lambda syntax. It's pretty ugly because of the temporary anonymous type, but that's how the let clause translates to lambda...

query =
    query.Select(item => new { item, itemTags = item.ItemTags.Select(it => it.TagName.ToLower()) })
         .Where(x => tagsList.All(t => x.itemTags.Contains(t)))
         .Select(x => x.item)
         .ToList();