PipelineStageDefinitionBuilderSet.cs

public class PipelineStageDefinitionBuilderSet<TDocument>
{
    private UpdateDefinition<TDocument> current;

    public PipelineStageDefinitionBuilderSet()
    {
        current = new BsonDocument();
    }

    public IPipelineStageDefinition ToPipelineStageDefinition()
    {
        var y = BsonDocument.Parse(
            """
                { $set : "FOO" }
            """);

        y["$set"] = current.RenderToBsonDocument();

        var pipe = new BsonDocumentPipelineStageDefinition<TDocument, TDocument>(y);
        return pipe;
    }

    public PipelineStageDefinitionBuilderSet<TDocument> Set<TField>(Expression<Func<TDocument, TField>> field, TField value)
    {
        var xs = Builders<TDocument>.Update.Set(field, value);

        var y = xs.RenderToBsonDocument();
        var temp = y["$set"].AsBsonDocument;

        current = Builders<TDocument>.Update.Combine(current, temp).RenderToBsonDocument();

        return this;
    }

    public PipelineStageDefinitionBuilderSet<TDocument> Push<TField>(Expression<Func<TDocument, IEnumerable<TField>>> field, TField value)
    {
        var xs = Builders<TDocument>.Update.Push(field, value);

        var y = xs.RenderToBsonDocument();
        var temp = y["$push"].AsBsonDocument.ElementAt(0);

        var d = BsonDocument.Parse(
            $$"""
            {
                   "{{temp.Name}}": { $concatArrays: [ "${{temp.Name}}", [ "FOO" ] ] }
            }
            """);

        d[temp.Name]["$concatArrays"][1][0] = temp.Value;

        current = Builders<TDocument>.Update.Combine(current, d).RenderToBsonDocument();

        return this;
    }

    public PipelineStageDefinitionBuilderSet<TDocument> Inc<TField>(Expression<Func<TDocument, TField>> field, TField value)
    {
        var xs = Builders<TDocument>.Update.Inc(field, value);

        var y = xs.RenderToBsonDocument();
        var temp = y["$inc"].AsBsonDocument.ElementAt(0);

        var d = BsonDocument.Parse(
            $$"""
            {
                 "{{temp.Name}}": { $sum: [ "${{temp.Name}}", "FOO" ] }
            }
            """);

        d[temp.Name]["$sum"][1] = temp.Value;

        current = Builders<TDocument>.Update.Combine(current, d).RenderToBsonDocument();
        
        return this;
    }
}

// https://stackoverflow.com/a/32146281
public static BsonDocument RenderToBsonDocument<T>(this FilterDefinition<T> filter)
{
    var serializerRegistry = BsonSerializer.SerializerRegistry;
    var documentSerializer = serializerRegistry.GetSerializer<T>();
    return filter.Render(documentSerializer, serializerRegistry);
}

public static BsonDocument RenderToBsonDocument<T>(this UpdateDefinition<T> filter)
{
    var serializerRegistry = BsonSerializer.SerializerRegistry;
    var documentSerializer = serializerRegistry.GetSerializer<T>();
    return filter.Render(documentSerializer, serializerRegistry).AsBsonDocument;
}