MongoDB .NET - Updating embedded document in list with filters based on Parent and Child

Question

Each Team document contains a list of Player documents. Each Player contains a list of strings named PlayerColors.

The need is to update the PlayerColors of a specific Player within the specific team. So I thought I could make one filter for the parent (Team) level and combine that with a filter for the child (Player) level.

But I haven't seen any examples that mirror this approach, so perhaps this is not possible?

var filterTeam = Builders<Team>.Filter.Eq("TeamName", "GoldDigger");

var filterPlayer = Builders<Player>.Filter.Eq("PlayerName", "Greg");

//   var combinedFilter = filterTeam & filterPlayer; // erroneous 

List<string> newColors = new List<string>() { "peach", "periwinkle" };

UpdateDefinition<Player> updateDefinition = Builders<Player>.Update.Set(doc => doc.PlayerColors, newColors); 

//   collection.UpdateOne(combinedFilter, updateDefinition);

A copy of this project is here in GitHub.

Yong Shun · Accepted Answer · 2025-10-10 00:19:26Z

As the Team document consists of Players array, you can't use the player filter with FilterDefinition<Player> type with the team filter with FilterDefinition<Team>.

What you need to achieve is with the $elemMatch operator.

Builders<Team>.Filter.ElemMatch(x => x.Players, filterPlayer)

Next same to the update definition, you need to provide the value with UpdateDefinition<Team>. Since you are updating the nested matching element(s), you need to looking for the filtered positional update operator $[<identifier>].

The MongoDB .NET Driver Fluent code for this update matching element is slightly complex, which I wrote the similar answer in MongoDB - How to filter and update on a child of a child.

In summary, you need:

AllMatchingElements(<identifier>) method in the updateDefinition
The UpdateOptions object with ArrayFilters property as the third arguments in the UpdateOne() method.

Full code:

var filterTeam = Builders<Team>.Filter.Eq("TeamName", "GoldDiggers");
var filterPlayer = Builders<Player>.Filter.Eq("PlayerName", "Greg");
var filterTeamPlayers = Builders<Team>.Filter.ElemMatch(x => x.Players,
    filterPlayer);

var combinedFilter = filterTeam & filterTeamPlayers;

List<string> newColors = new List<string>() { "peach", "periwinkle" };

UpdateDefinition<Team> updateDefinition = Builders<Team>.Update
    .Set(doc => doc.Players.AllMatchingElements("p").PlayerColors, newColors);

var updateResult = collection.UpdateOne(combinedFilter, updateDefinition,
    new UpdateOptions
    {
        ArrayFilters = new ArrayFilterDefinition[]
        {
            new BsonDocumentArrayFilterDefinition<Player>
            (
                new BsonDocument("p.PlayerName", "Greg")
            )
        }
    });

I had assumed the ElemMatch projection would only work for arrays, and not lists. Your comment also refers to arrays but my implementation was done only with Lists. However, it worked - so now I'm assuming the driver treats C# Lists as arrays.

Vaishali Dhokchawle · Accepted Answer · 2025-10-09 18:30:29Z

1

As two filters are of different types (here team and player) belong to different collection, we cannot combine those filters. So need to use two different filters- one to retrieve specific team(team filter) and second to get specific player using team filter.

answered Oct 9 at 18:30

Vaishali Dhokchawle

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