mongodb delete nested grandchild query in C#

Question

been trying for some time now to run a delete query against a mongodb collection; strongly typed objects, parents, nested children, each with nested grandchildren. i want to delete a specific grandchild, which is in a specific child, in a specific parent. i can get to MatchedCount = 1, but ModifiedCount is always 0 and the delete doesn't happen. i have tried many variations of the query, both from here and also using the AI engine on the mongo website. nothing works.

The entity structure is what you'd expect:

public class Parent
{
    public string Id {get; set;}
    public Child[] Children {get; set;}
}

public class Child
{
    public Guid Id {get; set;}
    public Grandchild[] Grandchildren {get; set;}
}

public class Grandchild
{
    public Guid Id {get; set;}
    public string Name {get; set;}
}

here is my latest attempt:

var filter = Builders<Parent>.Filter.Eq(x => x.Id, parentId);

var update = Builders<Parent>.Update.Pull(
  "Children.$[child].GrandChildren",
   Builders<GrandChild>.Filter.Eq(gc => gc.Id, Guid.Parse(grandChildId))
);

var arrayFilters = new List<ArrayFilterDefinition>
{
  new BsonDocumentArrayFilterDefinition<BsonDocument>(new BsonDocument("child._id", childId))
};

var updateOptions = new UpdateOptions { ArrayFilters = arrayFilters };

var result = await _parentCollection.UpdateOneAsync(filter, update, updateOptions);

return result.ModifiedCount == 1;

updating/deleting children is simple enough but i cannot get to the grandchild somehow. has anyone any idea what i am doing wrong?

EDIT:

in case anyone else is wrestling with similar - i now have a working solution to this, after 3 days (!!!) and some rehashing some of what i got from mongodb AI. i have used t-sql for 25 years and never suspected that something so simple could be so difficult to figure out. easy when you know how!

        var filter = Builders<Parent>.Filter.And(
            Builders<Parent>.Filter.Eq(p => p.Id, parentId),
            Builders<Parent>.Filter.ElemMatch(p => p.Children, c => c.Id == Guid.Parse(childId)));

        var update = Builders<Parent>.Update.PullFilter(
            "Children.$.GrandChildren",
            Builders<GrandChild>.Filter.Eq(gc => gc.Id, Guid.Parse(grandChildId))
        );

        var result = await _parentCollection.UpdateOneAsync(filter, update);

        return result.ModifiedCount == 1;

Try a recursive query : stackoverflow.com/questions/42787293/mongodb-recursive-query — jdweng
– jdweng, Commented Sep 4 at 12:24
thanks for your reply. i am not clear on how recursion would help in this case? my structure is strictly hierarchical. i need to go into the data once, to pinpoint what i am looking for and then remove it. — Aylmer Carson
– Aylmer Carson, Commented Sep 4 at 14:26
You have a parent, children, and grandchildren. Yes it is hierarchal, but you are having issues getting to the grandchildren which is why recursion is needed. — jdweng
– jdweng, Commented Sep 4 at 15:42
From Mongo themselves: "MongoDB does not support truly recursive queries for nested arrays," — Aylmer Carson
– Aylmer Carson, Commented Sep 4 at 16:16
Iterate over the "graph"; adding the entities (references) to a "to be deleted" collection. Now delete each entity in the "to be deleted" collection from the "target" collection ... in reverse order. — Gerry Schmitz
– Gerry Schmitz, Commented Sep 4 at 17:23

Rishit Bhatia · Accepted Answer · 2025-10-21 20:08:22Z

0

Glad you found a solution. The $elemMatch used helps find the specific grandchild here and the $pull operator helps remove it from the array of grandchildren. Another approach could've been:

//Delete a nested grandchild 
var parentId = "Parent1";
var grandchildIdToDelete = Guid.Parse("eff8318f-f464-4296-8ea7-67e670005ffb");
var filter = Builders<BsonDocument>.Filter.Eq("_id", parentId);
var update = Builders<BsonDocument>.Update.PullFilter("Children.$[].Grandchildren",
    Builders<BsonDocument>.Filter.Eq("_id", grandchildIdToDelete));
var result = _collection.UpdateOne(filter, update);
Console.WriteLine($"Modified Count: {result.ModifiedCount}");

answered Oct 21 at 20:08

Rishit Bhatia

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