How to Use $graphLookup to Find All Children in a Tree Structure in MongoDB?

Question

I have a tree structure stored in MongoDB where each node represents an object (e.g., folders, projects, documents, files). Each node has a list of ParentIds that reference its parent nodes. I want to use the $graphLookup aggregation stage to find all children (direct and indirect) of a given node by its _id.

public class TreeNode
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    public string Name { get; set; }

    public string ObjectType { get; set; }

    [BsonRepresentation(BsonType.ObjectId)]
    public List<string> ParentIds { get; set; } = new List<string>();
}

Test Data:

public async Task InsertNewTestData()
{
    if (await CheckIfDataExistsInCollection())
        return;

    var id1 = ObjectId.GenerateNewId().ToString(); 
    var id2 = ObjectId.GenerateNewId().ToString(); 
    var id3 = ObjectId.GenerateNewId().ToString(); 
    var id4 = ObjectId.GenerateNewId().ToString(); 
    var id5 = ObjectId.GenerateNewId().ToString(); 
    var id6 = ObjectId.GenerateNewId().ToString(); 
    var id7 = ObjectId.GenerateNewId().ToString(); 

    var objects = new List<TreeNode>
    {
        new TreeNode { Id = id1, Name = "Workspace", ObjectType = "folder", ParentIds = new List<string>() },
        new TreeNode { Id = id2, Name = "Project 1", ObjectType = "project", ParentIds = new List<string> { id1 } },
        new TreeNode { Id = id3, Name = "Document 1", ObjectType = "document", ParentIds = new List<string> { id2 } },
        new TreeNode { Id = id4, Name = "File 1", ObjectType = "file", ParentIds = new List<string> { id3 } },
        new TreeNode { Id = id5, Name = "File 2", ObjectType = "file", ParentIds = new List<string> { id3 } },
        new TreeNode { Id = id6, Name = "Document 2", ObjectType = "document", ParentIds = new List<string> { id2 } },
        new TreeNode { Id = id7, Name = "Project 2", ObjectType = "project", ParentIds = new List<string> { id1 } },
    };

    await _context.ObjectsTree.InsertManyAsync(objects);
}

Given this structure, how can I use the $graphLookup aggregation stage to find all children (direct and indirect) of a specific node by its _id? For example, if I provide the _id of "Project 1", I want to retrieve "Document 1", "File 1", "File 2", and "Document 2".

Here’s what I’ve tried so far, but it doesn’t seem to work as expected:

var pipeline = new[]
{
    new BsonDocument("$match", new BsonDocument("_id", new ObjectId("_id"))),
    new BsonDocument("$graphLookup", new BsonDocument
    {
        { "from", "ObjectsTree" },
        { "startWith", "$ParentIds" },
        { "connectFromField", "_id" },
        { "connectToField", "ParentIds" },
        { "as", "children" },
        { "depthField", "depth" }
    })
};

var result = await _context.ObjectsTree.Aggregate<BsonDocument>(pipeline).ToListAsync();

the problem here seems to be that the data for ParentIds is list of strings and it cannot search through them

{
  "_id": {
    "$oid": "679793f8e25c76dbf25bca3c"
  },
  "Name": "Project 1",
  "ObjectType": "project",
  "ParentIds": [
    {
      "$oid": "679793f8e25c76dbf25bca3b"
    }
  ]
}

and it cannot search through list of strings.

Answer 1

If you want to solve this with Linq and .NET classes instead of defining the aggregation stages in JSON, you need to derive a class from TreeNode that contains a property to store the children:

public class TreeNodeWithChildren : TreeNode
{
    public List<TreeNode> Children { get; set; } = new();
}

Afterwards, you can use the fluent interface to run the aggregation pipeline, e.g. (id2 denotes the id you are looking for):

var result = coll
    .Aggregate()
    .Match(x => x.Id == id2)
    .GraphLookup<TreeNode, TreeNode, string, List<string>, string, List<TreeNode>, TreeNodeWithChildren>(
        coll, 
        x => x.Id, 
        x => x.ParentIds, 
        x => x.Id, 
        x => x.Children)
    .ToList();