Command aggregate failed: Exceeded memory limit for $group, but didn’t allow external sort. Pass allowDiskUse:true to opt in

using System;
using System.Linq;
using System.Security.Authentication;
using MongoDB.Driver;
using MongoDB.Driver.Linq;

var settings = MongoClientSettings.FromUrl(new MongoUrl("mongodb://localhost:27017/test"));
settings.SslSettings = new SslSettings {EnabledSslProtocols = SslProtocols.Tls12};
settings.LinqProvider = LinqProvider.V3;
var mongoClient = new MongoClient(settings);
var mongoDatabase = mongoClient.GetDatabase("test");
var collection = mongoDatabase.GetCollection<OrderDao>("test");

var query1 = collection
    .AsQueryable()
    .SelectMany(i => i.Lines)
    .GroupBy(l => l.ItemId)
    .Select(g => new ItemSummary
    {
        Id = g.Key,
        TotalAmount = g.Sum(l => l.TotalAmount)
    });

var query1txt = query1.ToString();

Console.WriteLine(query1txt);
Console.WriteLine(query1txt.Contains("$push") ? "Uses $push :(" : "No $push here, hurray!");

var query2 = collection
    .AsQueryable()
    .GroupBy(l => l.Id)
    .Select(g => new ItemSummary
    {
        Id = g.Key,
        TotalAmount = g.Sum(l => l.TotalAmount)
    });

var query2txt = query2.ToString();

Console.WriteLine(query2txt);
Console.WriteLine(query2txt.Contains("$push") ? "Uses $push :(" : "No $push here, hurray!");

public class OrderDao
{
    public OrderLineDao[] Lines { get; set; }
    
    public decimal TotalAmount { get; set; }
    public Guid Id { get; set; }
}

public class OrderLineDao
{
    public decimal TotalAmount { get; set; }
    public Guid ItemId { get; set; }
}

public class ItemSummary
{
    public Guid Id { get; set; }
    public decimal TotalAmount { get; set; }
}

aggregate([{ "$unwind" : "$Lines" }, { "$project" : { "Lines" : "$Lines", "_id" : 0 } }, { "$group" : { "_id" : "$Lines.ItemId", "__agg0" : { "$sum" : "$Lines.TotalAmount" } } }, { "$project" : { "Id" : "$_id", "TotalAmount" : "$__agg0", "_id" : 0 } }])

No $push here, hurray!

aggregate([{ "$group" : { "_id" : "$_id", "__agg0" : { "$sum" : "$TotalAmount" } } }, { "$project" : { "Id" : "$_id", "TotalAmount" : "$__agg0", "_id" : 0 } }])

No $push here, hurray!

test.test.Aggregate([{ "$project" : { "_v" : "$Lines", "_id" : 0 } }, { "$unwind" : "$_v" }, { "$group" : { "_id" : "$_v.ItemId", "_elements" : { "$push" : "$_v" } } }, { "$project" : { "_id" : "$_id", "TotalAmount" : { "$sum" : "$_elements.TotalAmount" } } }])

Uses $push :(

test.test.Aggregate([{ "$group" : { "_id" : "$_id", "__agg0" : { "$sum" : "$TotalAmount" } } }, { "$project" : { "_id" : "$_id", "TotalAmount" : "$__agg0" } }])

No $push here, hurray!