C# MongoDb - How to update one field in nested documents?

Question

What I want to achieve is to provide List<JobList> containing client Id and InternalIds of jobs to the Finish() method. Then I want to iterate through clients and update all jobs matches JobInternalIds and set current datetime to FinishedAt field. The problem is that I totally don't know how to update nested objects.

I've tried something like below but without success.

public class Client
{
    [BsonId]
    public ObjectId Id { get; set;}
    public string Name { get; set;}
    public List<Job> Jobs { get; set;}
}

public class Job
{
    public string InternalId { get; set;}
    public string Name { get; set;}
    public DateTime? FinishedAt { get; set;}
}

public class JobList
{
    public string ClientId { get; set; }
    List<string> JobInternalIds { get; set; }
}

public async Task Finish(List<JobList> joblist)
{   
    var updateDefinition = new UpdateDefinitionBuilder<Client>()
        .Set(x => x.Job[0].FinishedAt, DateTime.UtcNow); // don't know how to set datetime to finishedAt for each elements of collection that matches InternalIds

    foreach(var item in joblist)
    {
        await _db.Collection.UpdateManyAsync(item.ClientId, updateDefinition);
    }   
}

----------EDITED----------
public async Task Finish(List<JobList> joblist)
{
    var updateDefinition = Builders<Client>.Update
        .Set(x => x.Jobs[-1].FinishedAt, DateTime.UtcNow);

    foreach (var item in joblist)
    {
        var internalIds = item.JobInternalIds.Select(x => x.InternalId).ToList();   
        var idFilter = Builders<Client>.Filter.Eq(x => x.Id, item.ClientId);
        var internalIdsFilter = Builders<Client>.Filter.ElemMatch(x => x.Jobs, y => internalIds.Contains(y.InternalId));
        var combinedFilters = Builders<Client>.Filter.And(idFilter, internalIdsFilter);

        await _db.Collection.UpdateManyAsync(combinedFilters, updateDefinition);
    }
}

Answer 1

You can use the filtered positional operator to update all elements that match a condition in an array:

var internalIds = new[] { "1", "2" };
var idFilter = Builders<Client>.Filter.Eq(x => x.Id, objectId);
var updateDefinition = Builders<Client>.Update
    .Set("Jobs.$[job].FinishedAt", DateTime.UtcNow);

await collection.UpdateManyAsync(idFilter, updateDefinition,
     new UpdateOptions
     {
         ArrayFilters = new []
         {
             new BsonDocumentArrayFilterDefinition<BsonDocument>(new BsonDocument("job.groupName", new BsonDocument("$in", new BsonArray(internalIds)))),
         }
     });

Check out this blog post for some examples - https://kevsoft.net/2020/03/23/updating-arrays-in-mongodb-with-csharp.html