EF LINQ - Return entities that contain an entire collection

Question

I am trying to troubleshoot the following LINQ Query:

public JsonResult SearchNodesByTags(string[] tags)
{    

    var nodes = _DbContext.Nodes.
            Where(n => n.Tags.All(t => tags.Contains(t.DisplayName)))
            .Select(n => new {n.NodeNativeId, n.NodeName, n.NodeClass.ClassName})
            .ToList();

    return Json(nodes);
}

The query is returning a single node that is not associated with a tag. What I want it to do, is return any nodes that have ALL the tags.

Answer 1

  .Where(n => n.Tags.All(t => tags.Contains(t.DisplayName)))

The way this is currently constructed, you're only going to end up with the Nodes where every tag in Node.Tags has a name in the tags whitelist, which includes Nodes with no tags.

---

You might want to use the answer from here on subsets:

_DbContext.Nodes
    .Where(n => !tags.Except(n.Tags.Select(t => t.DisplayName)).Any()) 
    .Select(...

• set1.Except(set2) contains elements of set1 that aren't in set2
• !set1.Except(set2).Any() == true if set2 includes every element of set1

---

Edit

It was pointed out in the comments that using Except could generate problematic queries, so I was thinking another option was to get a superset from the database, and further filter the objects within the application:

_DbContext.Nodes
    // filter nodes with any of the input tags
    .Where(n => n.Tags.Any(t => tags.Contains(t.DisplayName)))

    // select the information required
    .Select(n => new {
        n.NodeNativeId, 
        n.NodeName, 
        ClassName = n.NodeClass.ClassName,
        TagNames = n.Tags.Select(t => t.DisplayName) })

    // materialize super set with database
    .ToList()

    // filter so that only nodes with all tags remain
    .Where(n => !tags.Except(n.TagNames).Any())

    // produce result in anonymous class
    .Select(n => new { n.NodeNativeId, n.NodeName, n.ClassName })
    .ToList();

---

Edit 2

I just saw another one here that might work for you, but it requires that Tag.DisplayName is unique, since it could fail if you have multiple tags with the same DisplayName:

_dbContext.Nodes
     .Where(n => n.Tags.Count(t => tags.Contains(t.DisplayName)) == tags.Count)
     .Select(...