Using `$toLong` in MongoDB C# Queries

I'm struggling to convert the below Mongo aggregation pipeline into its C# equivalent. I am trying to convert ISODates to unix timestamps using the built in $toLong. There could be millions of documents, so doing .ToList().Select(x => ConvertToTimestamp(x.date)) is not feasible.

Say I have the following documents:

[
  {
    "value": 100,
    "date": ISODate("2022-03-26T04:38:28.044Z")
  },
  {
    "value": 50,
    "date": ISODate("2022-03-26T04:38:28.044Z")
  },
  {
    "value": 23,
    "date": ISODate("2022-03-26T04:38:28.044Z")
  },
  {
    "value": 199,
    "date": ISODate("2022-03-26T04:38:28.044Z")
  }
]

I have the following aggregation query that works

db.collection.aggregate({
  $project: {
    date: {
      $toLong: "$date"
    },
    value: 1,
  }
})

And I get the expected output

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "date": NumberLong(1648269508044),
    "value": 100
  },
  {
    "_id": ObjectId("5a934e000102030405000001"),
    "date": NumberLong(1648269508044),
    "value": 50
  },
  {
    "_id": ObjectId("5a934e000102030405000002"),
    "date": NumberLong(1648269508044),
    "value": 23
  },
  {
    "_id": ObjectId("5a934e000102030405000003"),
    "date": NumberLong(1648269508044),
    "value": 199
  }
]

Now, the question is, how to do this with the fluent aggregation API in C#?

Approach 1: Pass aggregation query as BsonDocument.

var pipelineDefinition = new BsonDocument[]
{
    new BsonDocument
    {
        {
            "$project", new BsonDocument
            {
                { "date", new BsonDocument { { "$toLong", "$date" } } },
                { "value", 1 }
            }
        }
    }
};

List<Model> result = _collection.Aggregate<Model>(pipelineDefinition)
    .ToList();

Approach 2: Work with BsonSerializer.

1. Implement IBsonSerializer class to convert from DateTime type to long type.

public class DateToLongConverter : IBsonSerializer<long>
{
    public Type ValueType => typeof(long);

    public long Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        var bsonReader = context.Reader;
        return bsonReader.ReadDateTime();
    }

    public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, long value)
    {
        var bsonWriter = context.Writer;
        bsonWriter.WriteInt64(value);
    }

    public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, object value)
    {
        var bsonWriter = context.Writer;
        bsonWriter.WriteInt64((long)value);
    }

    object IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        var bsonReader = context.Reader;
        return bsonReader.ReadDateTime();
    }
}

2. Apply BsonSerializer attribute with DateToLongConverter to Date property.

public class Model
{
    public ObjectId Id { get; set; }
    [BsonSerializer(typeof(DateToLongConverter))]
    public long Date { get; set; }
    public int Value { get; set; }
}

3. Register class map for Model class.

BsonClassMap.RegisterClassMap<Model>(cm =>
{
    cm.AutoMap();

    cm.MapMember(x => x.Date)
        .SetSerializer(new DateToLongConverter());
});

4. To query data:

List<Model> result = (await _collection.FindAsync(new BsonDocument()))
    .ToList();

> Demo